Background: Women with previous pre-eclampsia are at an increased risk of developing recurrent pre-eclampsia. Intervention with low dose aspirin had been recommended to reduce the incidence of recurrent pre-eclampsia. However, the association between interventions and maternal and neonatal outcomes in subsequent pregnancies in women with previous pre-eclampsia has not been fully studied.Methods: In this prospective study, a total of 41 patients with previous pre-eclampsia received low dose aspirin and active management , between 10 to 28 weeks until 32 to 34 weeks in our regional referral hospital. The recurrence of pre-eclampsia, and maternal and neonatal outcomes in this pregnancy were analyzed and compared to our previous study which reported a 60% recurrence of pre-eclampsia in our regional referral hospital.Results: Thirteen women with previous pre-eclampsia developed recurrent pre-eclampsia. The time of onset or severity of pre-eclampsia in the previous pregnancy was not associated with the incidence of recurrent pre-eclampsia. The time of onset of previous pre-eclampsia was also not associated with the time of onset in subsequent pre-eclampsia. However, the number of severe recurrent pre-eclampsia was significantly reduced, compared to their first pregnancies. The number of SGA and stillbirth/neonatal death was also significantly reduced in recurrent pre-eclampsia that was actively managed, compared to their first pregnancies.Conclusion: Despite the small sample size included in this study, our study demonstrates that active obstetric management reduces the incidence of recurrent pre-eclampsia, compared to our previous study, and reduces the severity of recurrent pre-eclampsia. It also improves neonatal outcomes in recurrent pre-eclampsia. However, because of no controls in this study, our findings need to confirmed by a case-control or randomized clinical trial study. Pre-eclampsia is a human-specific pregnancy disorder that is a leading cause of maternal and neonatal mortality and morbidity. It is clinically apparent only after 20 weeks of gestation or within the first 4–6 weeks postpartum and is responsible for about 70,000 maternal deaths and 500,000 fetal and neonatal deaths annually worldwide. Although the exact pathogenesis of pre-eclampsia is still unclear, a number of risk factors including maternal age, obesity and history of pre-eclampsia have been well-documented.Although some studies have reported that pre-eclampsia is more common in the first pregnancy, the incidence of recurrent pre-eclampsia in a subsequent pregnancy is around 13 to 65%  . To dateIt has been reported that clinical intervention with low dose aspirin, low molecular weight heparin, calcium supplementation, or vitamin D supplementation has some benefits for reducing the incidence of recurrent pre-eclampsia –8. StudiThis investigation conforms to the principles outlined in the Declaration of Helsinki. This prospective study was approved by the Ethics Committee of First Hospital of Xi'an Jiaotong University, China (ref 2018-115).in vitro fertilization (IVF) were also excluded.This prospective study was performed at one of the university teaching hospitals serving a diverse urban and rural population of approximately 8 million people in China. The First Hospital of Xi'an Jiaotong University is the main maternal care referral hospital in Xi'an city and large numbers of women with pre-eclampsia, in particular those women with high risk such as previous pre-eclampsia are referred to this hospital. A total number of 50 pregnant women with previous pre-eclampsia who came or referred to our high-risk obstetrics clinic were enrolled in this study between 2015 and 2018. All the women were randomly recruited. Women with multiple pregnancies, and underlying medical disorders such as pulmonary and cardiac disease were excluded from this study. Pregnancies conceived by Of the 44 enrolled pregnant women, two patients were lost during the follow-up in the third trimester and one patient had a spontaneous abortion at 19 weeks of gestation due to cervical incompetence. The remaining 41 patients gave a birth at our hospital and data were collected and analyzed.Data including maternal age, time of onset of previous and/or current pre-eclampsia, severity of previous and/or current pre-eclampsia, gestational week at delivery of previous and current pregnancies, neonatal results in previous pregnancy, interval between the two pregnancies, and pre-pregnancy BMI in this pregnancy were recorded by obstetricians.Pre-eclampsia was defined as a maternal systolic blood pressure ≥140 mmHg and/or diastolic blood pressure ≥90 mmHg measured on two occasions separated by at least 6 h, and proteinuria >300 mg in a 24 h period, or impaired liver function and lower platelet count, after 20 weeks of gestation in accordance with guidelines of the International Society for the Study of Hypertension in Pregnancy (ISSHP) in 2018  was defined birthweight under 10th percentile using the WHO fetal growth chart based on the gestational age (2) or obese (BMI > 27.50 kg/m2) at the time of first prenatal clinic to lose weight until their BMI neared 23 kg/m2 by monitoring their diet and exercise with guidance from a nutritionist.To reduce the fear and anxiety in this current pregnancy for patients who were recruited to this study, obstetricians in our high-risk clinical team shared the experiences of women with previous pre-eclampsia who did not develop pre-eclampsia in subsequent pregnancies. The experiences included the importance of antenatal visits or close monitoring including home blood pressure monitoring through the duration of pregnancy as well as obstetricians or midwives' contact. In addition, obstetricians also educated the family members, in particular the husband or partner about the risks of recurrent pre-eclampsia and encouraged family members to build a relaxed environment for pregnant women both at home and in the workplace. These pregnant women were asked to visit a prenatal clinic every 2–3 weeks, dependent on the maternal and fetal conditions until 34 weeks. After 34 weeks, they were asked to visit the prenatal clinic every week. Furthermore, because BMI is one of the main risk factors for pre-eclampsia, obstetricians recommended pregnant women who were overweight  between 10 weeks and 28 weeks of gestation until 32 to 34 weeks. Of them, 8 patients also received Low Molecular Weight Heparin (LMWH) (4100IU/day) when they were diagnosed with APS (antiphospholipid antibodies (aPLs) positive). The duration of treatment was up to 34 weeks of gestation for those who did not develop recurrent pre-eclampsia or up to 37 weeks of gestation or 12 h before delivery for those who developed recurrent pre-eclampsia. Patients were also given antihypertensive drugs such as labetalol or nifedipine to control the blood pressure when their systolic or diastolic blood pressure was over 130 or 80 mmHg measured by 24 h ambulatory blood pressure monitoring, even they have not developed pre-eclampsia yet.Prescribing antihypertensive drug to women with a history of pre-eclampsia, when their systolic or diastolic blood pressure is over 130 or 80 mmHg measured by 24 h ambulatory blood pressure monitoring, is our hospital protocol.Based on the results from the routine liver and renal function tests and echocardiography (ECG) monitoring, 24 h ambulatory blood pressure measurement or liver and kidney ultrasound were recommended to individual patients if it was necessary. In addition, since not all the patients stayed in the hospital, daily home blood pressure measurements were recommended to all patients in order to monitor the trend of blood pressure to see whether blood pressure increased or the maternal conditions deteriorated. At least, one senior obstetrician was able to be contacted during the study period, if women did not stay in the hospital.www.OpenEpi.com). P < 0.05 were considered significant.Data were presented as mean and standard deviation (SD) or number or percentage as appropriate. The statistical differences in maternal age, gestational week at diagnosis, gestational week at delivery, and stillbirth/neonatal death between first pregnancy and subsequent pregnancy were assessed by the Mann-Whitney U-test or Fisher's exact test using the Prism software package. The comparison between the severity or time of onset between first pre-eclampsia and recurrent pre-eclampsia was assessed by McNemar test using OpenEpi . The number of patients who had BMI ≥23 kg/m2 in the no recurrence group was not statistically different to that in the recurrent group . The mean start-time of aspirin intervention was significantly earlier in patients who did not develop recurrent pre-eclampsia compared to patients who did . The gestational weeks at delivery and birthweight were significantly earlier or lower in patients who developed recurrent pre-eclampsia than in patients who did not (p < 0.001), but the number of SGA infants was not statistically different between the two groups .Overall, of the 41 recruited patients in this study, 13 (32%) patients developed recurrent pre-eclampsia. The clinical parameters of patients who developed recurrent pre-eclampsia and those who did not are summarized in n = 19) who developed recurrent pre-eclampsia (n = 28) who developed recurrent pre-eclampsia  patients with previous severe pre-eclampsia , 9 (70%) patients developed early onset pre-eclampsia in the subsequent pregnancy and 3 (23%) patients developed severe pre-eclampsia. We next compared the time of onset or severity of pre-eclampsia in the first pregnancy with that of the subsequent pregnancy in these patients . A total of 4 (30%) stillbirth/neonatal death were recorded in their first pregnancy in patients with recurrent pre-eclampsia, which was significantly higher than the number of stillbirth/ neonatal death in subsequent pregnancy (0%) (p < 0.0001). There was also no difference in birthweight between the first and subsequent pregnancies in patients with recurrent pre-eclampsia .In patients with recurrent pre-eclampsia  women who received antihypertensive drugs. Of those that received the antihypertensive drugs, 10 women developed recurrent pre-eclampsia including 3 severe form and 7 mild form. The mean gestational age at delivery for these 3 women was 36+3, and the mean birthweight was 2,775 g. All these infants who were born from 3 women with severe pre-eclampsia were admitted to NICU. In contrast, in 16 women with a history of pre-eclampsia who did not receive antihypertensive drugs, 3 women developed recurrent pre-eclampsia, including 1 severe and 2 mild form.We also analyzed the postnatal outcomes. There was no stillbirth or neonatal death, and the mean birthweight was 2,775 g. The mean gestational age at delivery was 36In this prospective and observational study, we report that the incidence of recurrent pre-eclampsia was 32% in patients with a history of pre-eclampsia who received active management including low dose aspirin. We also found that severity or the time of onset of pre-eclampsia in the first pregnancy was not associated with the development of recurrent pre-eclampsia. In addition, early onset pre-eclampsia in the first pregnancy was not associated with early onset pre-eclampsia in the recurrent pre-eclamptic group. However, active management including the low dose of aspirin can reduce the rate of severe recurrent pre-eclampsia, compared to their first pregnancies.Although a history of previous pre-eclampsia does not predict the recurrence of pre-eclampsia in subsequent pregnancies, previous pre-eclampsia is one of the risk factors for the development of recurrent pre-eclampsia. Clinical intervention including low dose aspirin has been recommended by The American College of Obstetricians and Gynecologists (ACOG) and other guidelines to use in pregnant women with previous pre-eclampsia in order to reduce the incidence of recurrent pre-eclampsia , 12. AspPre-eclampsia is divided into severe and mild forms, or early onset and late onset forms according to the severity or the time of onset. It has previously been reported that women with early onset or severe pre-eclampsia in their first pregnancy are at an increased risk of developing recurrent pre-eclampsia , 14. HowIt is well-reported that the time of intervention is very important. ACOG recommends that the optimal intervention time is between 12 and 28 weeks of gestation . A recenPrevious studies have reported that even in the absence of recurrent pre-eclampsia, women with a history of pre-eclampsia are at a higher risk of delivering an SGA infant in their subsequent pregnancy . That stThere are some limitations in this prospective study. First, the sample size was relatively small, as this study was performed in a single maternal care referral hospital. Our study should be repeated with a larger sample size to confirm our findings including maternal and fetal outcomes in the future with multi maternal centers. Second, due to the clinical practice protocol in our hospital, we did not have a control group that did not receive active management. Third, some of the clinical parameters such as BMI in the first pregnancy were not available. Whether controlling weight gain in subsequent pregnancy had benefits on reducing recurrent pre-eclampsia needs to be studied in the future.Despite the small sample size included in this study, in this prospective study, we demonstrate that active management including administration of low dose aspirin starting from 10 to 12 weeks can reduce the incidence of recurrent pre-eclampsia in women with previous pre-eclampsia, compared to our previous study which reported 60% recurrent pre-eclampsia. Active management can also reduce the incidence of severe recurrent pre-eclampsia, reduce the risk of delivering an SGA infant and prevent stillbirth/neonatal death in patients with recurrent pre-eclampsia. However, there were no controls who did not receive active management in this study. Our findings need to be further confirmed by a case-control or a randomized clinical trial study.The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.The studies involving human participants were reviewed and approved by the Ethics Committee of First Hospital of Xi'an Jiaotong University, China (ref 2018-115). The patients/participants provided their written informed consent to participate in this study.XD and XL: involved in patient recruitment reported in this work. MH and SZ: data analysis. MT: involved in manuscript draft. XL and QC: designed study and wrote the manuscript draft. All authors were involved in the drafting, editing, and approval of the manuscript for publication.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


The platform enabled on-demand regime alternations within one sample run, in which the sub-channel regime generated smaller charged droplets. Ionization efficiencies for saccharides, glycopeptide, and proteins were enhanced in the sub-channel regime. Non-specific salt adducts were reduced and identified by regime alternation. Surprisingly, the sub-channel regime produced more uniform responses for a peptide mixture whose relative ionization efficiencies were insensitive to ESI conditions in previous picoelectrospray study. The nested channels also allowed effective washing of emitter tip for multiple sampling and analysis operations.On-demand electrospray ionization from different liquid channels in the same emitter was realized using filamented capillary and gas phase charge supply. The solution sub-channel was formed when back-filling solution to the emitter tip by capillary action along the filament. Gas phase charge carriers were used to trigger electrospray ionization from the solution meniscus at the tip. The meniscus at the tip opening may be fully filled or partially empty to generate electrospray ionization in main-channel regime and sub-channel regime, respectively. For emitters with 4 μm tip opening, the two nested electrospray (nested-ESI) channels accommodated ESI flow rates ranging from 50 pL min Nested electrospray ionization alternates on-demand between microscale main-channel and nanscale sub-channels. An existing challenge of ESI is that ionization efficiency of analyte is flow-dependent and sample-dependent.2 While accurate control and prediction of the ionization efficiency has not been achieved, ESI at lower flow rates has been found to provide higher analytical sensitivity, reduce ionization suppression and improve sample utilization efficiency.3,4 One critical aspect of lower flow rate is that it produces initial ESI droplets of smaller sizes that bear more charge per unit volume. Smaller charged droplets also require fewer fission events prior to the generation of gas phase ions.5 These provide sensitivity enhancement for analytes that are disfavored in charge competition during the evolution of ESI.6 Developments in the past three decades have brought methods to perform ESI at microflow, nanoflow, and more recently at picoflow.7–9 The sensitivity enhancement at lower flow rate has been demonstrated for saccharides,10 glycopeptides and glycoproteins,11 and antibodies.12 Beyond higher ionization efficiency, smaller charged droplets also play significant roles in eliminating non-specific adducts,13,14 and in the study of accelerated reaction kinetics.15Electrospray ionization (ESI) is a soft ionization method that produces intact molecular ions from solution phase samples.−1.16,17 However, pulsed ion sources are complicated by the possibility that the flow rates, droplet sizes and ionization efficiencies may vary at different time points in each pulse. For continuous ESI methods, a lower flow rate relies on the use of smaller emitter tips.18 20–40 nL min−1 was considered to be the lower limit of flow rate range for continuous self-fed ESI using heat-pulled micrometer-sized emitter tips.8,9 However, flow rates of submicron emitter tips and nanofluidic slot (50 × 300 nm) achieved 0.75 nL min−1,19 and 0.11 nL min−1 respectively.20 External pump-driven ESI could reduce flow rates for micrometer-sized emitter tip down to 0.3 nL min−1.9,21 Although a wealth of methods have been developed in the electrochemical field to manufacture nanopipette,22 the efforts of lowering the ESI flow rate by using smaller emitter tips have been constrained by practical obstacles such as emitter clogging,23 nanometer tip fabrication,24 and sample handling.13Pulsed DC and inductive AC electrospray has been demonstrated improved sample utilization efficiency with reduced flow rate to 30–160 pL min25 have successfully demonstrated electrospray from the meniscus at the edge of flat surfaces.26,27 If such surface-supported meniscus can be formed inside an emitter tip, it will be much smaller than the tip opening and ideal for generating ESI with lower flow rates and smaller droplets.Besides using the circular opening of the capillary tips to form ESI meniscus that covers the whole tip opening, other emitter configurations have been found to be effective as well to forming the ESI meniscus. Ambient ionization methods−1, defined as the sub-channel regime. When fully filled, the same emitter was demonstrated to also support nanoflow ESI up to 150 nL min−1, defined as the main-channel regime. The system allows rapid alternation between sub-channel and main-channel regimes in the same experiment, which reliably demonstrated the reduced initial droplet size and improved ionization efficiency in the sub-channel regimes. The presence of two separate channel also enabled the more complicated washing operations.Here we report a system and methods to generate ESI from meniscus smaller than the size of the emitter tip. The emitter was partially filled to perform continuous ESI at flow rates down to 50 pL min28 However, the partially filled tip associated with this individual liquid channel has not been utilized.Sample solution was delivered to an emitter tip by capillary action along an inner-filament (160 μm O.D. glass rod) annealed to the inner wall of a glass capillary (0.89 mm I.D.). In the taper region, while the glass capillary was heat-pulled to form an emitter tip of 4 μm O.D., the inner-filament was pulled to 160–180 nm O.D. In a typical backfilling, the solution migrates to the emitter tip to fill the tip and then the rest of the taper. During the process, there should be a transient moment when the solution initially arrives to partially fill the tip opening with a meniscus smaller than 4 μm. This type of inner-filament itself has been widely used to assist solution transport in capillaries for microelectrode, mass spectrometry, and patch clamp applications.29 and an electrode pusher was built to provide a steady flow of charges to the emitter. Description of the setup and plasma discharge is detailed in Fig. S1–S5, ESI section 1 and 2.30 Both regimes ionized compounds to the same types of typical ESI ions  ion signal needed for the investigation of ionization performances.In the sub-channel regime, the electrospray flow is limited by the capillary flow in the sub-channel. The capillary flow is a passive liquid delivery phenomenon that becomes dominant at microscale and nanoscale.9,10 Rapid alternation using the same emitter and sample is advantageous when carrying out repeated comparison experiments. On-demand alternations using arbitrary time pattern also help to rule out potential contributions from other effects that would have their own time profiles.One unique feature of nested-ESI is that it allows rapid alternation between main-channel and sub-channel regimes in the same experiment. The pusher electrode provides instantaneous electronic control over the charge supply that is independent of the solution flow. As illustrated in via evaporation and fission, the total amount of charge is determined at the generation of initial charged droplets. Smaller-sized initial droplets generated by lower flow rates and smaller emitters were found to significantly improve the ionization efficiency of saccharide analytes, which are known to have poorer surface activities than peptides and are suppressed under the typical electrospray ionization conditions.10,37 In this work, the generation of smaller initial charged droplet by the sub-channel regime was confirmed by the ionization behavior of an equal molar mixture of maltoheptaose (Mhep) and neurotensin (N). In the main-channel regime, the signal intensity ratio between Mhep and neurotensin resemble those obtained using wire-in nanoESI. When alternating into the sub-channel regime, an abrupt decrease of peptide ion signal was observed in synchrony with (1) the disappearance of solution in the main-channel and (2) the dimming of ESI plume. This sudden change aligns with the proposed rapid transition between two distinct channels and associated menisci. The sub-channel meniscus at the tip is believed to also adhere to the nm-sized inner-filament at the tip. Rough estimations suggest that the cross-section of the sub-channel meniscus at the tip could be 200–2000 times smaller than that of the 4 μm main-channel.Smaller initial charged droplets in the sub-channel regime were suggested by the optical observations during the alternation. Although ESI droplets could all evolve to become smaller 10 During the transitions into sub-channel regime, the peptide ion intensity and the total ion current (TIC) plummeted within 1 scan event, while the absolute ion intensity of Mhep did not decrease. To our surprise, the absolute intensity of Mhep would increase 2–5 folds, −1, the relative Mhep intensity gradually increased from 20% to 50% with the decreasing neurotensin intensity. When the flow rate was lower than 4 nL min−1, the sub-channel regime gave predominantly equal responses (100% ± 10%) for Mhep and neurotensin. This equal response is in the character of that obtained by wire-in nanoESI using a 100 nm emitter tip, which had much lower absolute intensities. In one experiment, the flow rate of the main-channel was always higher than that of the sub-channel. However, the higher sub-channel regime flow rates (up to 12 nL min−1) in one experiment could exceed the lower main-channel regime flow rates (down to 2–5 nL min−1) in another. The data points in Fig. S1530,38 was not observed in the experiments of this work.One surprising result of the sub-channel regime ESI is the increase of absolute ion intensity of saccharides. In previous low flow ESI researches, enhanced ionization efficiency was obtained at the cost of decreasing (30-fold) absolute ion intensities.m/z 365: angiotensin II m/z 524) of 0.020 and 1.3 were observed in the main-channel and sub-channel regimes, respectively, Fig. S16.n-octyl-glucopyranoside (OGP), a 10-fold increase of the relative ion intensity of turanose was observed in the sub-channel regime, Fig. S17.†Enhanced absolute ion intensity in the sub-channel regime was also observed for a few other saccharides. For lighter molecular weight maltose in a mixture with a peptide angiotensin II, a similar trend was observed. Peak intensity ratios  under all flow rates.9 When analyzing a similar peptide mixture by nested-ESI, the main-channel regime had relative peak intensities similar to those in Smith's work. In the sub-channel regime, the intensity of melittin ions increased by two orders of magnitude and became the base peak (100%), −1, which is higher than the 0.4 nL min−1 in Smith's work. This indicates the smaller meniscus in the sub-channel regime is more critical than the lower flow rate for the observed ESI phenomena. Although an exact uniform response was not yet achieved for the peptide ions, the more uniform relative intensities obtained in the sub-channel regime of this work suggest that the observed ionization efficiency of peptide is not solely analyte-dependent as previously believed. Chemical and physical environments of charged droplets can significantly impact ionization efficiencies. Ways to manipulate the charged droplets is still worth exploring for a deeper understanding of the ionization process, which may lead to better and eventually uniform ion response.Nested-ESI also exhibits different trends for more challenging analytes systems. Unlike saccharide–peptide mixtures,44 but opposite to those results by using the same emitter at lower flow rates.45 This again supports the presence and predominant roles of smaller ESI meniscus in the sub-channel regime. Additional effects from plasma ions are also suggested by the slightly higher charge states in the main-channel mode compared with those obtained by wire-in nanoESI, such as myoglobin in 46 One explanation for the observed ACS shift is that the plasma produced protonated water and solvent cluster in the surrounding air. This would reduce proton transfer from analytes ions to neutral gas phase solvent clusters. This kind of charge retention47,48 would lead to the observed higher charge states without denaturing the proteins.It was also found that peptides and proteins were ionized to higher averaged charge states (ACS) by the sub-channel regime. This trend is in accordance with the results by using smaller emitters,49 If this trend extend to sub-μm droplets, the ionized air produced by the plasma ions in this work would lead to a faster elimination of large residue droplets. This would be beneficial for evaporating droplets in the charge residue model, which could explain the significantly enhanced ionization efficiency for the proteins. One typical application of the smaller charged droplets is the reduction of salt clusters and non-specific adducts. Besides achieving these effects in the sub-channel regime, comparing the two sets of data from the rapid regime alternation allowed facile identification of non-specific adduct ions, Fig. S21, ESI section 13.It is worth noting that higher ionization efficiencies were observed for proteins in nested-ESI, particularly the sub-channel regime. This effect is evident in both absolute ion intensity, and intensity relative to the solution background ions, 50,51 shotgun lipidomics,52 and cell analysis.53,54 One limitation of direct nanoESI analysis is the carryover contamination. For nested-ESI, carryover contamination is localized in a much smaller region due to the largely empty emitter. Another unique feature of nested-ESI is that the emitter tip can draw solutions from both the sub-channel and the front end. By using the front end to load sample solution, while supplying blank solvent from the sub-channel, the emitter was washed during the nanoESI analysis (ESI section 14−1) was taken off from the platform for manual sampling. The manual operation took 15 s, in which time the main-channel was filled by 0.5 nL. Then, dipping this emitter tip into a sample solution for 9–12 s aspirated roughly 5 nL sample solution. Mounting the emitter back to the platform took another 30 s, during which time another 1 nL blank solvent would accumulate in the main-channel. Then the loaded sample was analyzed by nested-ESI. The ionization was initially in the main-channel regime, during which the inner surface was continuously washed by the flow of blank solvent from the sub-channel. 10 μM analytes were effectively washed before ESI enters sub-channel regime, as revealed by the full scan , before the tip opening of the emitter may be filled (main-channel). The two nested channels allow an emitter tip to accommodate a wider range of electrospray flow rates. In the sub-channel regime, the partially empty emitter tip, the fainter ESI plume, the enhanced saccharide ion signal and the higher average charge states all suggest that smaller initial charged droplets were produced from its meniscus.Separated controls over the exterior charge supply and interior solution flow allow the ESI to alternate between sub-channel and main-channel regimes on-demand. For the first time, this rapid alternation allowed repeated and reliable comparisons between ESI regimes. The sub-channel regime enhanced the ionization efficiency of hydrophilic compounds and proteins to much higher magnitudes. Non-specific adduct was reduced and identified by regime alternation. The nested two channels also allowed facile washing of the emitter tip during sample analyses.This work provides a new avenue to generate ESI with lower flow rates and smaller initial charged droplets. Particularly encouraging is the trend toward a more uniform ion response that was not observed in previous picoelectrospray study. This suggests the observed ion response is not just analyte-dependent and there is much more to be explored for the generation, manipulation and better understanding of ESI droplets.There are no conflicts to declare.SC-012-D0SC06221A-s001SC-012-D0SC06221A-s002


Transmission rates after exposure to a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)–positive individual within households and healthcare settings varies significantly between studies. Variability in the extent of exposure and community SARS-CoV-2 incidence may contribute to differences in observed rates.We examined risk factors for SARS-CoV-2 infection in a randomized controlled trial of hydroxychloroquine as postexposure prophylaxis. Study procedures included standardized questionnaires at enrollment and daily self-collection of midturbinate swabs for SARS-CoV-2 polymerase chain reaction testing. County-level incidence was modeled using federally sourced data. Relative risks and 95% confidence intervals were calculated using modified Poisson regression.Eighty-six of 567 (15.2%) household/social contacts and 12 of 122 (9.8%) healthcare worker contacts acquired SARS-CoV-2 infection. Exposure to 2 suspected index cases (vs 1) significantly increased risk for both household/social contacts  and healthcare workers . Increased contact time also increased risk for healthcare workers , but not for household/social contacts. County incidence did not impact risk.In our study, increased exposure to SARS-CoV-2 within household or healthcare settings led to higher risk of infection, but elevated community incidence did not. This reinforces the importance of interventions to decrease transmission in close contact settings. In an analysis of cases of SARS-CoV-2 transmission in a postexposure prophylaxis trial, increased exposure to SARS-CoV-2 within household or healthcare settings led to higher risk of infection, but higher community incidence did not. As of 7 January 2022, more than 300 million cases and 5.5 million deaths have been attributed to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) worldwide . Viral sIn studies of household and healthcare worker transmission, secondary cases of SARS-CoV-2 are typically attributed to the identified index case. However, in communities with high incidence, these secondary cases could alternatively have been community acquired. This misclassification would lead to inaccurate calculation of attack rates within households and healthcare settings. Systematic reviews of household transmission studies found that secondary attack rates vary widely from study to study, ranging from 4% to 55% . SecondaVariability in these studies may also arise from differential risk factors in the study population. Among household contacts, known risk factors for SARS-CoV-2 acquisition include age, contact with a symptomatic compared to asymptomatic case, and more frequent contact with an index case . HoweverIn a randomized clinical trial of the efficacy of hydroxychloroquine as postexposure prophylaxis for preventing SARS-CoV-2 infection, we observed an overall attack rate of 14% over 13 days among those who were not infected with SARS-CoV-2 at baseline . InformaBetween March and August 2020, we recruited participants from across the United States to take part in a double-blind, household-randomized controlled trial of hydroxychloroquine as postexposure prophylaxis to prevent SARS-CoV-2 acquisition after close contact with an infected person  . The stuWithin 2 days of study enrollment, participants received a box via courier containing study medication, sample collection supplies , instructions, and packing materials for return. Participants were instructed to collect a midturbinate swab at the time of study box arrival and daily for the next 13 days. The packaged swabs were collected by a courier at several time points throughout the participation period and shipped back to the University of Washington for reverse-transcription polymerase chain reaction (RT-PCR) testing. A transmission episode was defined as a PCR-detectable SARS-CoV-2 infection in a participant who was SARS-CoV-2 negative at baseline.The University of Washington Virology Laboratory conducted the RT-PCR testing using the primer sets N1 and N2 targeting the SARS-CoV-2 nucleocapsid . Assays County-level data on population size, daily number of COVID-19 cases and deaths, 7-day SARS-CoV-2 test positivity, and hospital inpatient bed use were sourced from the Health and Human Services data platform, HHS Protect . To esti10 daily county incidence rate) and all other treatment, demographic, socioeconomic, exposure, symptom status, and household characteristics of interest with a univariate P < .1, excluding characteristics with significant results for very small groups . Daily study incidence rates were calculated using the number of participants with their first positive PCR result on that calendar day divided by the number of participants at risk on that day. The smoothed study incidence rate curve and 95% CI were calculated using a Poisson generalized additive model of the number of positives with an offset for the log number at risk. Daily county incidence rates are the mean of the modeled rates in the county of each participant at risk on that day (the same average rate for the participant’s 14-day follow-up period is used for each day a participant is at risk). The smoothed county incidence rate curve was calculated using local regression weighted by the number at risk. Statistical analyses were performed in R software, version 4.0.To be included, participants needed to have a negative baseline swab. Household/social contacts and healthcare workers were analyzed separately. Relative risks (RRs) and 95% confidence intervals (CIs) were calculated using modified Poisson regression for binary outcomes with robust standard errors , accounting for within-household clustering. The multivariate models include the primary exposure of interest . Participant identification as Hispanic, Asian, or Black/African American was associated with higher risk of SARS-CoV-2 infection relative to participant identification as non-Hispanic White . Smoking was associated with a lower risk of SARS-CoV-2 infection with an RR of 0.25 . Having multiple suspected index cases was associated with a higher risk of SARS-CoV-2 infection: RRs, 2.01  for 2 suspected index cases and 2.23  for >2 suspected index cases.The multivariate model included 565 of the 567 participants from 455 of 456 households, excluding 2 participants with missing data. Variables included in the multivariate analysis were county incidence rate, participant race/ethnicity, participant smoking, number of suspected index cases, symptomatic index case (prior to the time of enrollment), living with index case in the 14 days prior to enrollment, and number of occupants per bedroom in the household . In the The mean modeled daily county incidence rate was 42.0  daily new infections per 100 000 residents during follow-up in the counties where participants were located. We enrolled 122 healthcare worker contacts who were negative for SARS-CoV-2 at baseline. The median age was 37 years , and 70% were female. The univariate RRs of SARS-CoV-2 infection in healthcare worker contacts by selected risk factors are displayed in Variables included in the multivariate model were county incidence rate, number of suspected index cases, and hours of exposure to an index case . Having The mean modeled county daily incidence rate was 29.8  per 100 000 residents during follow-up of healthcare worker contacts. In this national, multicenter prospective study, we found that increased exposure, but not local infection incidence, contributed to the risk of SARS-CoV-2 acquisition for household contacts. This epidemiological data supports the conclusion of many studies identifying household exposure as a primary source of risk, even when community incidence is relatively high. To our knowledge, our study provides one of the most detailed quantifications of factors that contribute to risk of household transmission with the most comprehensive postexposure testing. We identified multiple suspected index cases as the key independent risk factor for household transmission. The lack of association between the reported time spent with the index case and successful transmission could represent incomplete isolation of index cases in the household; indicate that small amounts of intense exposure, such as caregiving, pose a large risk; or that exposure prior to the test result, which might not have been captured in our survey, led to much of the transmission. All these potential possibilities suggest the need for more aggressive interventions to limit exposure when there is an index case identified in a household. The report of time spent could also be subject to recall bias, but the categories for analysis were wide, which would mitigate the effect of some inaccuracy.The lack of association between local incidence and study incidence confirms that an infection within the household posed far greater short-term risk than infection in the surrounding community and has implications for our understanding of how mitigation policies affect household spread. Prior studies have shown that containment measures such as lockdown, distance learning, telework, and closure of nonessential businesses decrease the proportion of SARS-CoV-2 infections acquired in the community and increase the proportion of infections acquired in the household, even though total number of infections may be lower , 18. HowAs expected, the time spent in contact with an index case was associated with risk of transmission for healthcare workers. Our finding about the importance of multiple exposures is novel. Although having >2 exposures was not statistically significantly associated with transmission, we suspect this can be attributed to the small sample size in this cohort. Little prior information exists regarding the effect of multiple high-risk exposures on risk of SARS-CoV-2 infection for healthcare workers, as this variable is not often included in studies . In the Interestingly, contact with a symptomatic index case did not increase risk of SARS-CoV-2 infection compared to an asymptomatic index case, contrasting with prior literature . Due to Participant identification as Asian also increased risk of SARS-CoV-2 infection in household/social contacts, which has not consistently been seen in prior studies . These pSome of the subgroups for healthcare workers contained very small numbers, including some categories of race/ethnicity, level of education, and housing type, making it difficult to draw conclusions about their effect on risk. The method of recruitment and completion of study procedures also likely selected for participants with a higher socioeconomic status, as is reflected in the high proportion of participants with more than a high school education. This may limit the generalizability of the findings to a more diverse population. Also, we analyzed county-level data for SARS-CoV-2 incidence rates due to the data available. However, finer-resolution data at the ZIP code or census tract level may have been more representative of an individual’s actual risk.In this national, multicenter study in the era prior to vaccine deployment and the emergence of highly transmissible variants, we found that regardless of changing community-level transmission or mitigation behaviors, limiting exposure to a known SARS-CoV-2–infected household member or patient remains a critical approach to interrupting transmission. Our novel study design incorporated community incidence into the model and found no significant association between this metric and the probability of transmission within households in the setting of a known close contact. However, widespread community mitigation measures present in most of the United States during the period of the study may have affected these findings, as contact with others outside of the household was limited. It is unclear at this time if the effect of community transmission on household transmission would change in the absence of mitigation measures, which currently vary across the country. As cases rise again due to variant emergence coinciding with increased in-person workplace and school interactions, more support for aggressive mitigation measures aimed at household and healthcare settings is critical to limit spread of SARS-CoV-2.


Theranostics is an emerging paradigm that combines imaging and therapy in order to personalize patient treatment. In nuclear medicine, this is achieved by using radiopharmaceuticals that target identical molecular targets for both imaging (using emitted gamma rays) and radiopharmaceutical therapy  for the treatment of various diseases, such as cancer. If the therapeutic radiopharmaceutical cannot be imaged quantitatively, a “theranostic pair” imaging surrogate can be used to predict the absorbed radiation doses from the therapeutic radiopharmaceutical. However, theranostic dosimetry assumes that the pharmacokinetics and biodistributions of both radiopharmaceuticals in the pair are identical or very similar, an assumption that still requires further validation for many theranostic pairs. In this review, we consider both same-element and different-element theranostic pairs and attempt to determine if factors exist which may cause inaccurate dose extrapolations in theranostic dosimetry, either intrinsic  or extrinsic  to the radiopharmaceuticals. We discuss the basis behind theranostic dosimetry and present common theranostic pairs and their therapeutic applications in oncology. We investigate general factors that could create alterations in the behavior of the radiopharmaceuticals or the quantitative accuracy of imaging them. Finally, we attempt to determine if there is evidence showing some specific pairs as suitable for theranostic dosimetry. We show that there are a variety of intrinsic and extrinsic factors which can significantly alter the behavior among pairs of radiopharmaceuticals, even if they belong to the same chemical element. More research is needed to determine the impact of these factors on theranostic dosimetry estimates and on patient outcomes, and how to correctly account for them. A therapeutic counterpart or “theranostic pair” radiopharmaceutical with a beta, alpha, or Auger electron emitting radionuclide may also be administered to deliver lethal radiation to the same regions of high uptake observed in the imaging procedure (e.g. tumors/metastases) Theranostics is a paradigm in which therapy and diagnosis are combined. In nuclear medicine, this is achieved by using radiopharmaceuticals that bind to the same molecular targets for both imaging and therapy 131I) for both the diagnosis and treatment of thyroid diseases. Recently, theranostics has formed a substantial research and industry paradigm with an extensive range of applications under development Theranostics emerged with the use of iodine-131  can only be quantified by its Bremsstrahlung emissions or very low positron emission. Actinium-225 (225Ac) could potentially be imaged due to its gamma emitting daughters (francium-221 (221Fr) and bismuth-213 (213Bi)), but due to the injected activities being very low, it is challenging to obtain good counting statistics in a clinically acceptable scan duration.Quantitative images, which are required for dosimetry, are difficult to obtain for some therapeutic radionuclides. For example, yttrium-90 (111In) and zirconium-89 (89Zr) have been used as surrogates to predict therapeutic absorbed doses or to perform retrospective dosimetry after therapy 177Lu)), or to a pharmaceutical that can be labeled with one radioisotope for imaging ) and with another for treatment (e.g. 225Ac). In this work we primarily use it to refer to a theranostic pair of radioisotopes, but we also discuss 177Lu in detail due to its importance in the field. Theranostic dosimetry with the use of an imaging surrogate can be performed using two different isotopes or chemical elements to label a given pharmaceutical. The validity of this approach has, to our knowledge, not been fully established, and needs to be validated for each theranostic pair.To overcome this, imaging nuclides such as indium-111  same-element (e.g. 86Y/90Y) same-pharmaceutical pairs and (ii) different-element (e.g. 111In/90Y) same-pharmaceutical pairs.We first explain theranostic dosimetry in detail and discuss sources that may lead to inaccurate dosimetry estimates. We present a non-inclusive list of pharmaceuticals along with theranostic radionuclides they are commonly labeled with and their targeted diseases, with a focus on oncology. We review and discuss intrinsic and extrinsic factors relating to the (i) chemistry of the radiopharmaceuticals, (ii) biological behavior in the body, and (iii) physics of the radionuclides that can alter We note that the most common use of theranostic imaging is in identifying patients suitable for RPT and assessing the treatment efficacy. Here, we focus on gathering understanding regarding the accuracy of theranostic dosimetry.The committee on Medical Internal Radiation Dose (MIRD) has proposed a schema using quantitative imaging to estimate radiation absorbed doses in RPT While organ-level dosimetry only provides the mean absorbed dose in the tissue of interest, voxelized dosimetry allows one to obtain a 3D absorbed dose distribution. Voxel-level dosimetry may use voxelized S-values or dose point kernels, which are scoring matrices of the radial absorbed dose distributions around a voxel or point source, respectively, of a given radioisotope, usually placed in water The current gold standard for dosimetry is the full MC method, which uses MC simulations of photon and particle transport within anatomically realistic phantoms (derived from patient computed tomography (CT) images), combined with the activity distribution from patient-specific PET or SPECT images, to obtain voxelized absorbed dose estimates in tissues of interest In theranostic dosimetry, the surrogate's TAC , is decay corrected using the physical half-lives of the imaging and therapeutic radionuclides to predict the TAC of the therapeutic radiopharmaceutical. Theranostic dosimetry estimates are not only useful to prevent toxicity but can also be used to determine optimal activities for subsequent therapy cycles.177Lu is theranostic on its own as it can be imaged quantitatively with SPECT and the beta particles from its decay are useful for treatment. Regardless, an imaging surrogate may still be beneficial for pre-therapy dosimetry for nuclides like 177Lu. In this context, “pre-therapy dosimetry” refers to dosimetry done prior to therapy, either to determine an optimal injected activity for therapy, or to predict the therapeutic absorbed doses.The popular radionuclide Two primary assumptions are made when utilizing the imaging surrogate approach for dosimetry:(i) The radiopharmaceuticals have similar biodistributions (i.e. the degree of accumulation in different organs in the body at a given timepoint) even if variables such as the injected amount of each radiopharmaceutical, the physiology of the patient, or the expression level of the target pre, intra, or post-therapy differ.(ii) Both radiopharmaceuticals have similar pharmacokinetics  in the tissues of interest.If these two assumptions are not met, the extrapolated TAC from the surrogate will not match the therapeutic radiopharmaceutical's TAC and incorrect absorbed doses will be obtained. Figure While the TACs generally differ when the imaging and therapeutic radiopharmaceuticals are different biological targeting vectors, it is often assumed that switching a therapeutic radioisotope/nuclide to an imaging one using the same tracer backbone does not alter the biological behavior of the radiopharmaceutical in any way. However, this is not guaranteed to be true. It is therefore important to assess the validity of this statement for various theranostic pairs and to understand the uncertainties in absorbed dose prediction with this approach.Several components of a radiopharmaceutical can be changed: the chemical element, the isotope of the given element, the pharmaceutical, and/or the chelator ). In this work, we primarily discuss changing only the element or isotope. We focus on 3 different classes of theranostic pairs:Same-element isotope pairs” in which the imaging and therapeutic radioisotopes belong to the same chemical element and they are both used to label the same chemical compound. Tables 86/90Y]Y-DOTATOC 124/131I]I-Metaiodobenzylguanidine (mIBG) for neuroblastomas 1) “Isotopes have the same number of electrons and protons, the only difference being the number of neutrons. The chemical and biological behavior of an element is mainly determined by its electronic structure, and so it is assumed that isotopes of the same chemical element have identical intrinsic chemical behavior Different-element pairs” in which the imaging and therapeutic radionuclides belong to different chemical elements and are both used to label the same chemical compound. Tables 64Cu) labeled DOTATATE ([64Cu]Cu-DOTATATE) and [177Lu]Lu-DOTATATE for NETs in vivo biological behavior in the body, but it is often assumed they do for treatment planning and dosimetry.2) “Different-pharmaceutical pairs” in which different radiopharmaceuticals that target the same molecule are labeled with radioisotopes or radionuclides of the same or different elements. For example, [68Ga]Ga-PSMA-11 (PSMA being prostate specific membrane antigen) and [177Lu]Lu-PSMA-617 99mTc) labeled macroaggregated albumin (MAA)/90Y microspheres.3) “The biological behavior of a radiopharmaceutical is a critical component for subsequent dosimetry estimates. Before we look at this in more detail, we want to highlight a few points:(i) Estimated absorbed dose calculations, even when not following the imaging surrogate approach but using the therapeutic radiopharmaceutical itself, may already only be accurate within 10-20% in the best case scenario (ii) Some effects discussed in the following may require more investigation to be fully understood. As such, it may be difficult to isolate the true causes of some of these phenomena. For instance, if the pharmacokinetics of a radiopharmaceutical is altered post-therapy, we may not know if this is due to damage at a cellular level, a difference in injected amounts pre- vs. post-therapy, the saturation of receptors, or any other possible effect. Furthermore, most of the time, if not all, it is likely a combination of many factors that impact the biological behavior of a radiopharmaceutical.(iii) If the difference in behavior between two radiopharmaceuticals is known precisely, we could potentially correct for it, e.g. by applying a scaling factor to account for differences in biodistributions between the imaging and therapeutic radiopharmaceutical. Therefore, while this review aims to uncover behavioral differences between radiopharmaceutical pairs, the presence of differences does not necessarily mean that an imaging surrogate should not be used or that dosimetry estimates are guaranteed to be inaccurate. More research is required to determine the precise impact of behavioral differences on dosimetry estimates.(iv) Ultimately, the most important goal of dosimetry is to improve patient outcomes. If pre- or post-therapy dosimetry with an imaging surrogate improves patient outcomes compared to no dosimetry or only dosimetry with the therapeutic radiopharmaceutical, theranostic dosimetry should still be performed even if large differences in biological behavior between the radiopharmaceuticals are discovered, as long as toxicity is not a possible side effect.Below, we discuss general factors that can cause differences in the biodistributions and/or the pharmacokinetic behavior of radiopharmaceuticals. We include both intrinsic  and extrinsic (e.g. injected amount) factors that may result in those differences.18F) labeled sodium fluoride (NaF) (a bone seeking agent) and [18F]FDG  respectively The biological behavior of a radiopharmaceutical can change via the so-called “mass effect”, which causes a change in uptake in certain organs depending on the injected mass or amount (e.g. the molar amount or the activity). This effect is dependent on the binding site. Easily saturable sites such as cell surface-bound PSMA receptors can only accommodate a finite number of bound ligands; the uptake will not change regardless of how much more radiopharmaceutical is injected This effect is well understood for certain pharmaceuticals; for example NHL patients treated with ibritumomab tiuxetan  are injected with unlabeled rituximab before therapy to block the CD-20+ binding sites on B-cells in non-target organs, enabling better accumulation of Zevalin on tumor cells 177Lu]Lu-NeoBOMB1 (a GRPR antagonist) resulted in a 1.3 times higher tumor absorbed dose and a 5 times lower pancreas absorbed dose compared to an injection of 10 picomoles. The tumor and pancreas clearance half-lives of [177Lu]Lu-NeoBOMB1 also changed  and [111In]In-J591 (an anti-PSMA antibody) with increasing injected mass 111In]In-J591 from 10 mg to 100 mg increased the residence time in the whole body, serum, and tumor lesions of patients by 10 minutes to 29 hours (depending on the region), and decreased it in the liver by 11 hours A pre-clinical study in which mice were injected with 200 picomoles of Lu-DOTATATE is 7.4 GBq 68Ga]Ga-DOTATATE is used for imaging 177Lu]Lu- and [68Ga]Ga-DOTATATE 177Lu- and 68Ga-labeled DOTATATE respectively, a difference of > 15-fold.These effects are likely due to the tumor and target organs expressing significantly more receptors than the non-target tissues. A higher target-to-background ratio is beneficial for visualization and treatment purposes but may create issues for theranostic dosimetry. The injected amount of the imaging radiopharmaceutical is almost always much lower than its therapeutic counterpart. For example, the standard injected activity in a therapeutic cycle of Lu-PSMA-617 has been shown to have an effective half-life of 33.0 hours in the kidneys 68Ga is only 68 minutes. An effective half-life of 33.0 hours corresponds to 11.6 half-lives of 68Ga, at which point sequential quantitative imaging is likely not able to capture the biological half-life of PSMA-617.Statistics decrease as the imaging radionuclide decays, and image quality may become poor, potentially leading to less reliable quantitative data and inaccurate TACs (though the extent of this depends on scanner sensitivities and performances). The radionuclide may even decay before the radiopharmaceutical has finished its retention or elimination stage in tissue. For example, [123I]I-mIBG  and [131I]I-mIBG  Furthermore, the uptake of a radiopharmaceutical is not instantaneous. It first circulates in the blood, the rate of uptake in tissues depending on factors such as blood flow rate, degree of binding to blood components, and permeability across capillaries As discussed in Section 2, one could decay correct the imaging radiopharmaceutical's TAC to attempt to obtain the therapeutic radiopharmaceutical's TAC  and  are the activities of the therapeutic and imaging radiopharmaceuticals at point x, y, z at time t respectively, and and  are the physical half-lives of the therapeutic and imaging radionuclides respectively. This leads to a more accurate dosimetry estimate. However, if the biological half-life of the radiopharmaceutical is long and the physical half-life of the imaging surrogate is short, the predicted washout kinetics may be incorrect. According to MIRD Pamphlet No. 16, ignoring the long-term slow washout component of the TAC can result in errors in the residence time calculation of up to 45% where If the physical half-life of the imaging radionuclide is much shorter than that of the therapeutic radionuclide, it might decay before the pharmacokinetics needed to replicate the therapeutic radiopharmaceutical are completely captured. This would result in an incorrectly predicted TIA which would lead to an incorrect absorbed dose estimate. If the true washout curve of the therapeutic radiopharmaceutical is unknown, it may be impossible to correct for this.Chemical differences between radiopharmaceuticals play a large role in determining their biological behavior. This is primarily an issue between different-element and different-pharmaceutical pairs, as same-element isotope pairs are assumed to have the same intrinsic chemical behavior 177Lu, 68Ga, 90Y, 225Ac, etc.) are called coordination compounds. The coordination complexation chemistry of the radiometal into the chelator will vary depending on the radiometal. For example, 68Ga3+ forms a hexadentate (N4O2) complex with DOTA in octahedral geometry, where two carboxylate groups remain free 177Lu3+ ion forms an octadentate complex with DOTA, which means there are no free carboxylate groups, shown in Figure Pharmaceuticals labeled with a radiometal  can result in high liver uptake in mice 68Ga]Ga-PSMA resulted in increased vascular activity in the patient If the radiopharmaceutical is unstable, the radionuclide may become uncomplexed in the body and roam freely. This can be caused by demetallation , radiolysis (the dissociation of molecules caused by the radiation itself) or by the enzymatic degradation of peptides. For example, demetallation of 177Lu]Lu-DOTATATE, Lutathera, is conjugated with DOTA, excess DTPA is added to chelate any free 177Lu ions that remain in the mixture as they accumulate in bone and can result in significant absorbed dose 177Lu]Lu-DOTATATE or [177Lu]Lu-DTPA, the total body retention was 20% or 4% of the injected amount respectively There are also radiopharmaceutical dependent chemical differences that need to be considered. While the FDA approved In-octreotide, there was a 10-fold higher binding of labeled octreotide in blood serum 20 months after therapy due to the presence of octreotide antibodies The pharmacokinetics of a radiopharmaceutical in a patient may change intra- or post-therapy because of various biological effects, i.e. the loss of tumor cells after therapy or the formation of antibodies against the tracer, which has been observed in some patients treated with octreotide and rituximab 90Y]Y-DOTATOC doubled the residence time of [111In]In-octreotide in many organs nine weeks post-therapy, significantly increasing the biological effective dose (BED)  Table . If postAltered behavior of a radiopharmaceutical post-therapy is most important when using dosimetry to determine optimal injected activities for subsequent cycles of therapy. With each cycle of therapy, the initial dosimetry estimate may become less accurate as pharmacokinetic changes occur. While pre-therapy dosimetry may enable more accurate prediction of dosage for the first therapy cycle, these results highlight the importance of reassessing dosimetry after each cycle to track pharmacokinetic changes and the importance of personalized dosimetry in general. Each patient is likely to respond differently to therapy and may or may not reach absorbed dose limits to organs-at-risk. Without personalized dosimetry, there is no way to assess these differences. We do note that this effect is also important outside of dosimetry with an imaging surrogate, when using the therapeutic radiopharmaceutical to determine absorbed doses after the first therapy cycle.The pharmacokinetics and/or biodistribution of the radiopharmaceutical can change in between cycles due to effects such as the creation of antibodies against the tracer. This can result in over/undertreatment if the dosage is based on dosimetry prior to or after the first therapy cycle only. At this time, it is unclear how to account for this effect because it likely varies wildly amongst patients. The imaging surrogate is also subject to post-therapy pharmacokinetic changes, which highlights the importance of performing dosimetry after each cycle.The specific and molar activities of a material  are defined as the radioactivity per mass (Bq/g) and per mole (Bq/mol) respectively. Depending on how the radionuclide and radiopharmaceutical are manufactured, they may contain traces of the stable radionuclide (known as the carrier) and/or other chemical impurities. Only some radionuclides are truly carrier-free, and they are still susceptible to contamination when the final radiopharmaceutical is being synthesized. The SA of a radiopharmaceutical is usually reported, although it is not considered during dosimetry estimates.Lowering the specific or molar activity of a radioisotope/nuclide decreases its incorporation into the pharmaceutical during radiolabeling. Radiolabeling is performed by direct labeling methods or by the use of a chelator 68Ga]Ga-DP11 (a PSMA inhibitor) by 16-fold did not significantly change the uptake in the tumors of mice, but resulted in a 6-fold lower renal uptake 131I]I-mIBG had a 1.25 to 2 times lower uptake in organs such as the adrenals, liver, kidneys, thyroid, bone, etc. at certain timepoints compared to 148-185 MBq/mg SA [131I]I-mIBG, while there were no significant difference in tumor uptake 111In]In-pentetreotide Decreasing the SA of FDG scans reduced the mean myocardial SUVmax by 1.9 times 123I]I-mIBG Patient behaviors such as diet, exercise, smoking/drug use, and the use of certain medications can all alter a radiopharmaceutical's biodistribution and/or kinetics Patient behavior can impact dosimetry in different ways depending on the length of time between the administration of the imaging and therapeutic radiopharmaceuticals. To our knowledge, for most therapies there are no prescribed guidelines for how long to wait between administering each radiopharmaceutical, but ranges such as 8 days apart for mIBG  and 7-9 Although not exactly a patient behavior, dosimetry estimates may be impacted by concomitant medications given during therapy such as amino acid infusions to reduce renal uptake. Amino acids and other positively charged molecules can reduce renal doses by 20-55% It is difficult and maybe impossible to ensure that the radiopharmaceutical biokinetics will be identical for the imaging and therapy scans. Patients may have gained or lost weight, started or stopped medications, etc. The position of the patient may also differ between each scan, which makes image registration difficult; however methods have been proposed to account for this The biological behavior of a radiopharmaceutical in a given patient may change on a day-to-day basis due to changes in diet, drug/medication use, etc. If imaging and therapy are performed at different times, differences in the biodistributions or radiopharmaceutical kinetics could impact dosimetry estimates. These differences can be reduced by administering the imaging surrogate as close to the therapeutic injection as possible, but this is not always possible due to factors like receptor saturation. Obvious behavioral changes in factors like diet, smoking, or drug use should attempt to be minimized in between imaging and therapy.We will now discuss a few of the theranostic pairs listed in Tables 177Lu, a beta emitter that has shown excellent therapeutic efficacy with DOTATATE 177Lu also emits gamma photons and can be imaged with SPECT. The ability of 177Lu to be imaged quantitatively and be used therapeutically allows it to be a theranostic powerhouse.Before this, we would like to discuss the ideal scenario, which is that the therapeutic radionuclide can be imaged quantitatively during therapy. Currently, the most popular nuclide for this is 177Lu is still beneficial for pre-therapy dosimetry. For pre-therapy imaging, a lower half-life nuclide is preferable to reduce absorbed dose . The higher sensitivity of PET also allows the acquisition of higher quality images  compared to 131I (8.02 days). The TAC of [123I]I-mIBG may only capture the blood clearance phase of the radiopharmaceutical, while images of [131I]I-mIBG can capture the longer retention in the body that is more relevant for dosimetry [123I]I-mIBG dosimetry and intra-therapy [131I]I-mIBG dosimetry found a low correlation between organ absorbed doses, with a coefficient r value of 0.30 for the heart wall and liver, 0.60 for the lung, and 0.52 for the total body  123I]I-mIBG were more conservative than those with [131I]I-mIBG, which could result in undertreatment if post-therapy dosimetry is not performed Pre-therapy I- and [131I]I-L19-SIP (a mAb targeting the extra domain B of fibronectin) have been compared in preclinical studies with mice . Fielding et al. performed dosimetry with Tc-MAA and 90Y SPECT/PET , ranging from good correlations to poor 99mTc]Tc-MAA compared to 90Y SPECT Studies show large variations in agreement between absorbed doses/activity distributions between Tc-MAA uptake Many of these issues are difficult to avoid, and at this time dosimetry may not be as accurate if based on the MAA distribution. Correlations with biological response, which may be the most important factor, have also been conflicting, with some studies showing treatment response does not correlate with [99mTc]Tc-MAA is required regardless to determine the percentage of lung shunting, the activity concentration of 90Y is higher in this treatment compared to other RPTs with 90Y due to the direct injection into the hepatic artery. Therefore, dosimetry with 90Y may be more accurate during radioembolization than in other RPTs in which the concentrations are lower.While pre-therapy imaging with Y-DOTATOC has still been used to obtain dose-effect relationships for [90Y]Y-DOTATOC therapy The reduction in quantitative accuracy due to prompt gammas is more severe for 99mTc]Tc-MAA may be a poor surrogate for 90Y microspheres, due to the nature of the radioembolization administration procedure and the differences between MAA and glass/resin microspheres. 90Y may be a better choice for liver and tumor dosimetry during radioembolization therapies, due to the higher activity concentrations. 111In also does not seem to be a good imaging surrogate for 90Y because of the chemical and biological differences between yttrium and indium labeled pharmaceuticals. Despite the possible PET imaging issues and differences in physical half-life, 86Y is likely a better imaging surrogate for 90Y. Because of the challenges of imaging 90Y, performing dosimetry with 86Y is likely more accurate than with 90Y at this time, however this still requires more study.Lu-PSMA I&T was found to be higher than [68Ga]Ga-PSMA I&T in certain tissues of tumor-bearing mice, such as the tumors, kidneys, and the spleen 68Ga]Ga- and [177Lu]Lu-DOTAZOL  had the same skeletal retention in mice, but [177Lu]Lu-DOTAZOL had a faster blood clearance and an approximately 7 times higher kidney uptake than [68Ga]Ga-DOTAZOL at 1 hour post-injection The %ID/g of Sc-PSMA-617 has been used for pre-therapy dosimetry of [177Lu]Lu-PSMA-617 in patients in vivo kinetics to [177Lu]Lu-PSMA-617 than [68Ga]Ga-PSMA-11 44Sc]Sc-PSMA-617 and [177Lu]Lu-PSMA-617 were found to have nearly identical hydrophilic properties, binding affinities to PSMA, and a similar tissue distribution profile in mice Several clinical studies have attempted to correlate standard uptake values (SUVs) from 64Cu]Cu-DOTATATE  was recently FDA approved in the US, allowing it to be used clinically for imaging NETs. [64Cu]Cu-DOTATATE was found to detect more NET lesions in patients than [68Ga]Ga-DOTATATE [177Lu]Lu- and [67Cu]Cu-DOTA-triglycine-F(ab′)2 (chCE7 antibody fragments which bind to L1-CAM for various cancers) have been compared in tumor-bearing mice, and the %ID/g of the 177Lu-labeled conjugate was found to be twice as high in the kidneys at 24 hours post-injection compared to the 64Cu-labeled conjugate (34.5 %ID/g versus 16.0 %ID/g). The reduction in copper uptake was thought to be due to the negative charge of the Cu-DOTA complex compared to the neutral Lu-DOTA complex 64Cu]Cu-DOTATATE). The kidneys are a dose-limiting organ in NET therapies, and an underestimated absorbed dose could result in kidney toxicity.The biological behavior of [177Lu]Lu- and [64Cu]Cu-cetuximab  were found to have similar uptake rate constants (the probability that the molecule will be trapped in the tissue due to antibody-antigen linking) in esophageal squamous cell carcinoma tumors, but different release rate constants (the probability the molecule will be released from the target), with coppers being higher by 5-fold Despite differences in catabolism and complexation chemistry, Zr-N-sucDf-cetuximab, 1.0 %ID/g for [177Lu]Lu-p-SCN-Bz-DOTA-cetuximab, and 2.0 %ID/g for [177Lu]Lu-p-SCN-Bz-DTPA-cetuximab 89Zr 18F has been proposed as a surrogate for 177Lu, particularly with PSMA binding tracers 177Lu using 18F has not been published. 18F has significant chemical differences to 177Lu; fluorine is not a radiometal and requires different pharmaceuticals and labeling strategies than 177Lu. For example, PSMA-617 cannot be labeled with 18F but it can be labeled with 177Lu. DCFPyL is a PSMA targeting molecule that can be labeled with 18F providing advantages for PET imaging. However, DCFPyL has a different structure than PSMA-617 Finally, 18F and 177Lu may be more appropriate. Several compounds have been developed including DOTA-[18F]-AMBF3-PSMA 18F/natLu for imaging, and natF/177Lu for therapy. This is an exciting development as the conformation of the pharmaceutical and its properties are expected to be the same for each application. This would not bypass issues caused by half-life differences, but it would open the door to an exciting new area of research with significant potential for theranostic applications.The use of a single pharmaceutical that can be labeled simultaneously with 177Lu, with the exception of 44Sc, although this may be due to a lack of research comparing 177Lu- and 44Sc-labeled pharmaceuticals. 68Ga might not be the best choice as a surrogate for 177Lu, because of chemical differences and significant physical half-life differences. However, 68Ga has shown some correlation between PET measured SUVs and 177Lu therapeutic absorbed doses. 64Cu may underestimate absorbed doses to organs-at-risk such as the kidneys, depending on the chelator used. As a [64Cu]Cu-DOTA labeled pharmaceutical was recently FDA approved, theranostic dosimetry with this radionuclide may become a feasible alternative. However, more research to understand the differences between [64Cu]Cu-DOTA and [177Lu]Lu-DOTA labeled pharmaceuticals and the errors in absorbed dose predictions should be done before it is adopted into common practice. 89Zr is promising as at this time significant biodistribution differences have only been observed in bone, however because of this care needs to be taken when estimating bone marrow absorbed doses. Finally, new developments into pharmaceuticals which can be labeled with both 18F and 177Lu simultaneously may open the door to exciting new theranostic applications.All of the imaging surrogates discussed here showed differences in biological behavior compared to 225Ac for RPT 225Ac could in principle be imaged with SPECT because of the gamma emissions of its daughters, injected activities are low to avoid toxicity which creates a challenging situation for quantitative imaging. Here, we will discuss 177Lu, 111In and 89Zr as potential surrogates. 177Lu and 111In can be imaged with SPECT, while 89Zr is a PET radionuclide.Recently, there has been great excitement for the use of alpha emitters such as 225Ac arise regardless of the choice of surrogate. Three of the daughters of 225Ac are alpha emitters, shown in Figure Some dosimetry issues with 213Bi  221Fr and astatine-217 (217At) in vivo is more difficult to determine in vivo kinetics 225Ac's daughters are being evaluated using SPECT ex vivo counting of samples, for example using gamma counters Pharmaceuticals which are internalized and stored in the cell rapidly after binding  along with the injected mass 225Ac]Ac-DOTATOC means dosimetry with 177Lu could severely underestimate the healthy tissue absorbed dose.[Ac- and 17LuLu-DOT225Ac in the original sample 225Ac]Ac-PSMA-617 has been quoted as > 98% 225Ac labeled to DOTA in general has been questioned. The DOTA chelator thermodynamically prefers smaller ions with larger metal centers resulting in less stable complexes, while Ac3+ (actinium's most stable oxidation state) is the largest of all known tripositive ions 225Ac from DOTA in vivo, which may make high liver uptake difficult to avoid The high liver uptake was thought to be due to uncomplexed 89Zr and 111In are both possible surrogates for 225Ac, although there has not been enough research to confidently analyze them at this time. However, studies have shown similar biodistributions between 225Ac- and 89Zr-labeled ofatumumab, an anti-CD20 antibody for NHL 111In has been used for pre-clinical dosimetry of 225Ac, labeled to hTAB004 (a murine antibody targeting tMUC1) for triple-negative breast cancers 111In]In-DTPA-anti-PD-L1-BC and [225Ac]Ac-DOTA-anti-PD-L1-BC  have been compared in tumor-bearing mice and pharmacokinetic differences in certain tissues were found, with 111In having higher %ID/g in the kidneys and tumor, and 225Ac having twice as high %ID/g in the liver at the 24 hour timepoint. However, this difference was hypothesized to be due to the differences in catabolism and clearance of the DTPA and DOTA chelates 225Ac's daughters make dosimetry challenging, regardless of the choice of surrogate. Current research indicates there may be significant biodistribution differences between 225Ac and 177Lu labeled to certain pharmaceuticals which could result in absorbed dose underestimation. There has not been enough research into the use of 89Zr and 111In as surrogates to make conclusions on their suitability. Because of the potential toxicity of 225Ac, theranostic dosimetry may be advantageous even if large differences in biodistributions are found, so long as the doses to healthy tissues are not underestimated.The recoil of 64Cu and 67Cu is a promising option for RPT and theranostics because of copper's versatile coordination chemistry, allowing it to be bound to many small molecules, antibodies and proteins 67Cu emits beta particles for therapy and gammas suitable for SPECT imaging, while 64Cu can be imaged with PET. 67Cu has shown similar therapeutic efficacy to 177Lu in mouse studies for prostate cancer 131I for NHL radioimmunotherapy The isotope pair 67Cu has been used for intra-therapy dosimetry of itself, such as with BAT-2IT-1A3 (an anti-CD20 antibody for lymphoma) 64Cu could be beneficial for pre-therapy dosimetry; [64Cu]Cu-Octreotate has been used for theranostic dosimetry of [67Cu]Cu-Octreotate in patients  67Cu and 64Cu can be found in Table 2. Yumi Sugo et al. 64Cu]CuCl2 in tumor-bearing mice using low SA [67Cu]CuCl2 to that calculated by Qin et al. 64Cu]CuCl2. Similarly, the biodistributions of [64/67Cu]Cu-RPS-085 (a PSMA binding ligand) were similar at 4 and 24 hours post-injection when different masses were injected (31 pmol versus 17 pmol respectively) In certain organs, copper metabolism is independent of SA 67Cu- and 64Cu-labeled pharmaceuticals, the research that exists is positive and suggests that 64Cu would be suitable for theranostic dosimetry of 64Cu, although we assume this pair is still subject to the external factors presented in Section 4.While there has not been much research comparing 67Cu- and 64Cu-labeled pharmaceuticals, the results available at this time are promising and indicate that 64Cu could be a suitable imaging surrogate for 67Cu.While there are not many studies comparing the differences between 152Tb and 155Tb have been proposed as imaging surrogates for the therapeutic 149Tb and 161Tb. The terbium isotopes are interesting to explore because of their diverse decay characteristics and their high treatment efficacy. Both [161Tb]Tb-PSMA-617 and [149Tb]Tb-PSMA-617 have been shown to be more effective than [177Lu]Lu-PSMA-617 for prostate cancer in mice 161Tb delivers higher absorbed doses to water density spheres than 67Cu, 47Sc, and 177Lu in MC simulations The terbium isotopes 161Tb can be imaged with SPECT which has been done in patients with DOTATOC 149Tb with PET, which has been done in mice with PSMA-617 161Tb emits both beta particles and Auger electrons and can be used for dual beta/Auger therapy 149Tb is an alpha emitter. 149Tb does not have alpha emitting daughters, which eliminates the potential toxicity from uncomplexed daughters after recoil, such as what occurs with 225Ac. However, the daughters of 149Tb are long-lived (5+ days), which would need to be considered during dosimetry estimates Both of the therapeutic radioisotopes can be imaged for intra-therapy dosimetry. 152Tb and 155Tb are suitable for PET and SPECT imaging, respectively. In vivo images in mice of each isotope have been obtained with good results 152Tb-DOTANOC 152Tb-PSMA-617 The surrogates To our knowledge, biodistribution and pharmacokinetic comparisons between terbium isotopes labeled to identical pharmaceuticals have not been compared, so we cannot comment on the suitability of the terbium isotopes as imaging surrogates. However, we expect they are subject to the same external factors as other same element isotope pairs, discussed in Section 4.152Tb as a PET imaging surrogate for 177Lu (in replacement of 68Ga). 152Tb has a longer half-life (17.5 hours) than 68Ga (68 minutes) and more similar chemical properties to 177Lu when labeled to PSMA-617 177Lu and 152Tb are both lanthanides An interesting possibility is the use of 152Tb and/or 155Tb would be good imaging surrogates for their therapeutic counterparts, 149Tb and 161Tb. However, we expect they behave similarly to other same-element isotope pairs discussed in this work and would likely be suitable imaging surrogates.More research is needed to identify if Dosimetry is a crucial step in the personalization of RPT. While the injected activities in patients is usually standardized or scaled by bodyweight 131I, 177Lu, 67Cu, 149Tb, and 161Tb can be imaged quantitatively, enabling personalized dosimetry calculations. Intra-therapy dosimetry using the therapeutic radiopharmaceutical should be encouraged whenever possible, as this eliminates most, if not all, of the sources of error discussed in this work. Whether or not to use the therapeutic radiopharmaceutical or an imaging surrogate should be assessed on a case-by-case basis, as the decision depends on many factors, such as the imaging quality, the half-lives of both radionuclides, the pharmaceutical, and the target.Many of the therapeutic radionuclides discussed in this work, such as 90Y  can impact the binding affinity, the biodistributions, and/or residence times in a given tissue, which is particularly important if only using one scan to perform dosimetry for subsequent therapy cycles. Differences in physical half-lives can cause an inability of the imaging surrogate to capture the washout kinetics of the therapeutic radiopharmaceutical.If these extrinsic and intrinsic effects become more well understood they may be corrected for, such as through the use of a scaling factor. However, many of these factors are dependent on the pharmaceutical, target site, and/or the radionuclides, so there is no “one-size-fits-all” way of accounting for these differences. Because the biological behavior of a radiopharmaceutical varies on a patient-by-patient basis, a scaling factor would only be entirely accurate if it was also obtained on a patient-by-patient basis, which would be difficult if not impossible to obtain; a population-based scaling factor would increase the accuracy but not be 100% accurate. Some effects like post-therapy effects are harder to account for because they vary heavily among patients Accounting for differences in physical half-life may not be possible. If the imaging surrogate decays before capturing the therapeutic radiopharmaceutical's washout pharmacokinetics, the true kinetics may never be known . As stated in Section 4.2, ignoring the long-term slow washout component of the TAC can result in errors in the residence time calculation of up to 45% 123I and 131I.Based on our analysis of specific theranostic pairs, we predict that most same-element isotope pairs are suitable for theranostic dosimetry. It is not guaranteed that the biodistributions and pharmacokinetics of the radiopharmaceuticals of the pair will be identical due to the extrinsic factors we have discussed. However, the extent of these differences is not precisely known. We believe, however, that the potential benefits of personalized dosimetry outweigh the differences. An exception to this, as stated, is if the physical half-life of the imaging surrogate is too short, such as with 111In versus 90Y Many different-element pairs appear to have significant intrinsic differences that may result in over/underestimation of absorbed dose, such as difference in bone marrow uptake of Something that is important to address is the extent to which the issues discussed in this work impact dosimetry calculations. As mentioned previously, personalized dose assessments, even under optimal conditions, have errors that can be higher than 20% Furthermore, even if there are significant differences in biological behavior, another question, perhaps the most important, should be considered: does dosimetry with an imaging surrogate result in better patient outcomes compared to not performing dosimetry at all, or compared to performing dosimetry only with the therapeutic radiopharmaceutical? If pre-therapy dosimetry with an imaging surrogate results in better patient outcomes, it should still be performed even if large biological differences between the surrogate and the therapeutic radiopharmaceutical are discovered, so long as there is not the potential for toxicity if the absorbed doses are underestimated.To this end, an optimal procedure may be to perform pre-therapy dosimetry with an imaging surrogate, and intra-therapy dosimetry with the therapeutic radiopharmaceutical (if it can be imaged) after each cycle to optimize injected activities in subsequent cycles. For therapeutic radiopharmaceuticals which cannot be imaged, performing dosimetry with an imaging surrogate post-therapy should be also encouraged if it results in better patient outcomes.At this time, personalized dosimetry is not routinely performed. This is vastly different from external beam radiotherapy (EBRT), in which every patient is given a detailed treatment plan to guide their therapy. It would be unthinkable in EBRT to administer therapy to a patient without a thorough treatment plan. Nonetheless, individual treatment plans are not common in RPT. They require multiple SPECT/CT or PET/CT scans, which are limited by cost, resources, and difficulty. Furthermore, for many patients, returning to the clinic for up to 4 scans after each cycle of therapy can be a large burden. In these cases, the initial diagnostic scan may be all that is available. The dose-response relationship is also significantly different for RPT than EBRT, for reasons such as heterogeneous dose distributions  and the much lower absorbed dose rate in RPT While research is ongoing, there are ways to reduce some of the burdens associated with dosimetry. Single time point dosimetry may allow accurate dosimetry estimates with less SPECT/CT and PET/CT scans. If complete dosimetry data (3 time points) are obtained after the first cycle of therapy, using a single time point at 24 hours after subsequent therapeutic injections only leads to a 2% difference in renal absorbed dose compared to taking a full data set after each cycle There has not been much research into the clinical impact of dosimetry with an imaging surrogate compared to dosimetry with the therapeutic radionuclide only. However, more work needs to be done to increase the prevalence of personalized dosimetry in general, and one way to do that is through theranostic dosimetry. As stated by Michael Stabin, “treating all nuclear medicine patients with a single, uniform method of activity administration amounts to consciously choosing that these patients be treated with a lower standard of care than patients who receive radiation externally for cancer treatments.” To this end, it is important to correlate theranostic and non-theranostic dosimetry with patient outcomes to ensure the highest possible standard of care and to determine the impact of the extrinsic and intrinsic factors we have discussed here on actual clinical and preclinical studies. We believe if care is taken to consider differences in biological behavior and other aspects discussed in this work between radiopharmaceuticals labeled with different pairs of isotopes, theranostic dosimetry would be immensely beneficial for patient care.This review was an attempt to answer the question: is it possible to accurately determine absorbed dose estimates for a therapeutic radiopharmaceutical by extrapolation from its imaging counterpart? There can be large differences in the biological behavior between identical pharmaceuticals labeled with different radioisotopes or radionuclides. Variables such as the injected amount/mass, physical-half-life, coordination chemistry, specific activity of the radiopharmaceutical, presence of chemical impurities, and patient behavior can all alter the biodistribution and pharmacokinetics of a radiopharmaceutical. These differences may potentially cause errors in absorbed dose estimates when using one radiopharmaceutical to predict the absorbed dose deposited by another radiopharmaceutical, even if the isotopes are of the same chemical element. Based on existing studies, same-element isotope pairs seem to be suitable for theranostic dosimetry; the benefits of dosimetry outweigh the differences in the biological behavior except when the physical half-life of the imaging surrogate is very short. The behavioral differences between many different-element pairs may be too large and result in significant errors in absorbed dose estimates, which could lead to over- or under-treatment. More research needs to be done analyzing the impact of these differences on dosimetry on a case-by-case basis for different radiopharmaceutical pairs. More efforts also need to be devoted to correlating patient outcomes with theranostic dosimetry, to determine if performing dosimetry with an imaging surrogate improves outcomes even if there are large behavioral differences between the radiopharmaceutical pairs.