In Vivo EPID Dosimetry Detects Interfraction Errors in 3D-CRT of Rectal Cancer

BACKGROUND In vivo dosimetry can record the delivered dose during radiotherapy, which may be used to trigger adaptive radiotherapy or other user intervention. We demonstrate the use of our in-house in vivo electronic portal imaging dosimetry in quantifying interfraction dose variability in rectal cancer. METHODS We recorded MV images from nine treatment beams for six patients prone on the belly board, during 4-7 fractions, for a total of 50 measurements. Images were processed with our dosimetry system to produce dose maps. The dose map from the reference fraction was compared to all subsequent ones to determine interfraction delivery variation, yielding 41 dose difference maps. RESULTS We identified a number of dose discrepancies. In several patients, persistent gas bubbles may result in cumulative dose deviations large enough to warrant adaptive radiotherapy. In three patients, discrepancies in dose resulted from variability of patient positioning on the belly board. These issues were not readily identified by standard imaging procedures. CONCLUSION We are developing an open-source in vivo portal dosimetry method to automatically track delivered dose at every fraction. Results can be used to flag unexpected discrepancies, guide adaptive radiotherapy, or warrant image guidance. Further data is needed to test applicability with other treatment sites and setups.