Estimations of relative biological effectiveness of secondary fragments in carbon ion irradiation of water using CR-39 plastic detector and microdosimetric kinetic model.
PURPOSE
To estimate relative biological effectiveness (RBE) ascribed to secondary fragments in a lateral distribution of carbon ion irradiation. The RBE was estimated with the microdosimetric kinetic (MK) model and measured linear energy transfer (LET) obtained with CR-39 plastic detectors.


METHODS
A water phantom was irradiated by a 12 C pencil beam with energy of 380 MeV/u at the Gunma University Heavy Ion Medical Center (GHMC), and CR-39 detectors were exposed to secondary fragments. Because CR-39 was insensitive to low LET, we conducted Monte Carlo simulations with Geant4 to calculate low LET particles. The spectra of low LET particles were combined with experimental spectra to calculate RBE. To estimate accuracy of RBE, we calculated RBE by changing yield of low LET particles by ± 10% and ± 40%.


RESULTS
At a small angle, maximum RBE by secondary fragments was 1.3 for 10% survival fractions. RBE values of fragments gradually decreased as the angle became larger. The shape of the LET spectra in the simulation reproduced the experimental spectra, but there was a discrepancy between the simulation and experiment for the relative yield of fragments. When the yield of low LET particles was changed by ± 40%, the change in RBE was smaller than 10%.


CONCLUSIONS
An RBE of 1.3 was expected for secondary fragments emitted at a small angle. Although, we observed a discrepancy in the relative yield of secondary fragments between simulation and experiment, precision of RBE was not so sensitive to the yield of low LET particles.
['Hirano|Yoshiyuki|Y|', 'Kodaira|Satoshi|S|', 'Souda|Hikaru|H|', 'Osaki|Kohei|K|', 'Torikoshi|Masami|M|']
D002244:Carbon D063193:Heavy Ion Radiotherapy D007700:Kinetics D018499:Linear Energy Transfer D008962:Models, Theoretical D009010:Monte Carlo Method D019047:Phantoms, Imaging D011092:Polyethylene Glycols D000072232:Radiation Dosimeters D011880:Radiotherapy Planning, Computer-Assisted D012062:Relative Biological Effectiveness D015203:Reproducibility of Results
secondary fragments CR-39 microdosimetric kinetic model RBE heavy ion therapy