SU‐E‐T‐177: Radiochromic Film: Towards a Public Calibration Function

Purpose: To assess the possibility of using a public calibration function for radiochromic film dosimetry in dose QA of highly conformal treatment plans. Methods: EBT3 film calibration strips (3.5×20cm 2 from lots A101212 and A011713) were exposed on a Varian Trilogy at a facility to a 10×10cm 2 open field at doses of 80,160,320 cGy using 6MV photons. Together with a strip of unexposed film from the same lot the exposed films were digitized in a single scan using different Epson 10000XL scanners at two different facilities. The dose‐response data for each color‐channel from each facility were generated using the same calibration function X(D) = a + b/(D‐c), where X(D) is the response at dose D and a, b and c are the coefficients. Different batches of EBT3 film were exposed to a VMAT beam. These films, plus two reference strips exposed to doses of zero and 160 cGy, were digitized on the scanners at the two facilities. Using the multi‐channel dosimetry method and One‐scan protocol (Med Phys, 39:6339‐49, 2012) the recorded doses on the VMAT films were calculated and the results were compared with the VMAT plan using a Gamma index of 3%/3mm. Results: The passing rates obtained for dose maps calculated for all combinations of VMAT images and calibration functions were nearly unchanged, using the One‐scan protocol. Also, in all cases a passing rate of >99% was obtained for Gamma index of 3%/3mm. On the other hand, if the One‐scan protocol was not employed, the dose maps for VMAT images and calibration functions from different scanners showed poor correlation with the treatment plan. This is probably due to the scan‐to‐scan variability. Conclusion: We have found that it is feasible to use a public calibration function for a given radiochromic film lot using the same methodology, One‐scan protocol, for patient‐specific QA. Two of the authors are the employees of Ashland Inc., the manufacturer of GafChromic film.