Technical Note: Intrafractional changes in time lag relationship between anterior-posterior external and superior-inferior internal motion signals in XXXX sites.

@entity5 @entity1 @entity788
PURPOSE: To investigate constancy, within a treatment session, of the time lag relationship between implanted markers in @entity5 and an external motion surrogate. METHODS: Six gastroesophageal junction and three @entity788 @entity1 (IRB-approved protocol) received two cone-beam CTs (CBCT), one before and one after treatment. Time between scans was less than 30 min. Each @entity1 had at least one implanted fiducial marker near the @entity5 . In all scans, abdominal displacement (Varian RPM) was recorded as the external motion signal. Purpose-built software tracked fiducials, representing internal signal, in CBCT projection images. Time lag between superior-inferior (SI) internal and anterior-posterior external signals was found by maximizing the correlation coefficient in each breathing cycle and averaging over all cycles. Time-lag-induced discrepancy between internal SI position and that predicted from the external signal (external prediction error) was also calculated. RESULTS: Mean    standard deviation time lag, over all scans and @entity1 , was 0.10    0.07 s (range 0.01-0.36 s). External signal lagged the internal in 17/18 scans. Change in time lag between pre- and post-treatment CBCT was 0.06    0.07 s (range 0.01-0.22 s), corresponding to 3.1%    3.7% (range 0.6%-10.8%) of gate width (range 1.6-3.1 s). In only one @entity1 , change in time lag exceeded 10% of the gate width. External prediction error over all scans of all @entity1 varied from 0.1    0.1 to 1.6    0.4 mm. CONCLUSIONS: Time lag between internal motion along SI and external signals is small compared to the treatment gate width of abdominal @entity1 examined in this study. Change in time lag within a treatment session, inferred from pre- to post-treatment measurements is also small, suggesting that a single measurement of time lag at the session start is adequate. These findings require confirmation in a larger number of @entity1 .
