SU‐E‐T‐142: Evaluation of the Energy Dependence of Gafchromic EBT3 Film for Electron Beams

Purpose: To evaluate the energy dependence of Gafchromic EBT3 film for establishing a quality assurance method of bolus electron conformal radiotherapy. Methods: We irradiated electron beam to EBT3 films, which were set in the water tank. The linear accelerator used was Varian Clinac 21EX. The energy of electron beams were 9 and 12 MeV. The irradiated field size was 10x10 cm² and the source to surface distance was 100 cm. The depths of measurement were 22 (depth of dose maximum; dmax), 31, and 37 mm for 9 MeV and 28 (dmax), 43, and 50 mm for 12 MeV. The irradiated doses were 25, 50, 75, 100, 150, 200, and 300 cGy. EBT3 films were readout with a flat‐bed scanner 48 hours after irradiation, and the optical density (OD) curve was obtained for each beam energy and depth. The OD curves were approximated by a third‐order polynomial. The doses were evaluated at netOD 0.1 and 0.3 from the approximated curves. Results: The differences of the evaluated doses from those for 9 MeV at 22 mm depth were from 2 to 14 % for netOD=0.1, and from 1 to 13 % for netOD=0.3, respectively. The netOD curves of dmax for both energies showed good agreement, while large discrepancy was found in the deeper depths. Conclusions: The dependence of dose response of EBT3 film on electron beam energy was small at dmax, while it increased at deeper depth in the present study. It can be considered that the discrepancy was caused by setup error because dose gradient was steeper at the deeper region. In future work, we will perform more precise measurement with a solid phantom to evaluate the energy dependence of EBT3 film.