Beam quality and dose rate dependency of Gafchromic EBT3 film irradiated with therapeutic megavolt photon beams

Abstract In the present study, the energy and dose rate dependence of Gafchromic EBT3 film (lot no. 03171402) irradiated with megavolt photon beams (i.e., 60 Co, 6MV, 10 MV and 15 MV) are evaluated for absorbed dose range i.e., 0.1 - 20 Gy. For flattening filtered beam, the average percentage deviation in film sensitivity normalized with 6 MV photon beam is 14.9 and - 6.5 respectively, for Co 60 and 15 MV, respectively at absorbed doses ≤ 1 Gy. Whereas for absorbed doses > 1 Gy, the average deviation of 3.9 % and -7.2 % is obtained for Co 60 and 15 MV, respectively. The average deviation for 10 MV (unfiltered) beam normalized with 6 MV unfiltered beam is found to be 4.4 % and 1.0 % respectively, for doses ≤ 1Gy and > 1Gy. The film sensitivity (OD/Gy) decreases from 0.114 to 0.014, 0.152 to 0.014, 0.146 to 0.014, 0.118 to 0.014 and 0.141 to 0.014, respectively for Co 60 , 6 MV, 6 MV (unfiltered), 10 MV(unfiltered) and 15 MV photon beam,which accounts for a decrease of 80-90% with increase of dose over the given range. The average relative sensitivity for absorbed doses ≤ 1 Gy are 0.90 i.e., a % variation of ~10.9 % and 0.999, i.e., deviation of only 0.1 % for absorbed doses > 1 Gy are obtained for irradiation with 6 MV photon beam (filtered and unfiltered beam), respectively at low (570MU/min) and high dose rate (1780 MU/min). The sensitivity of Gafchromic EBT3 film decreases with increase in dose and difference among all beam energies are significantly larger at a low dose (≤ 1Gy) as compared to a higher dose (>1Gy). The response of EBT3 Gafchromic film for the megavolt photon beams used in radiotherapy are energy and dose rate dependent at low dose region, whereas, at high absorbed dose, its response is independent of beam quality and dose rate.