Comparative analysis of the responses of EBT-XD and EBT4 films to ultra-high dose rate proton beams

Commercial radiochromic films, more specifically EBT-model Gafchromic films (EBT films), have the potential for two-dimensional profile measurements of ultra-high dose-rate (UHDR) radiation beams designed for FLASH radiotherapy. However, there is a controversy in regard to the dose-rate dependency of EBT-film responses to UHDR protons. In this study, we irradiated two types of currently available EBT films, EBT-XD and EBT4, to spread-out brag peak proton beams at 9 Gy at 180 Gy s -1 and 720 Gy s -1 in parallel and vertical beam incidences. The irradiated films were scanned with a flatbed scanner, and the inverted red color intensities were converted to absorbed doses based on the dose response curves of the red color component measured with LINAC 6 MV X-rays. The obtained dose levels were 5–15% lower than the delivered doses, indicating the quenching effects along the tracks of high-LET charged particles. The depth dose profile obtained with the EBT-XD film at 720 Gy s -1 was notably higher than that at 180 Gy s -1 , whereas such a difference was not observed with EBT4. The enhanced response of the EBT-XD film was confirmed by the sectional beam profiles obtained using vertically incident beams. These results imply that radiochromic reactions in EBT-XD film can be enhanced at the center of a Gaussian-shaped proton beam under certain UHDR conditions. Further studies are necessary to clarify the mechanism underlying the change in the responses of EBT films to UHDR protons. • The dose profiles of two UHDR proton beams were measured using EBT-XD and EBT4 films. • Similar dose profiles were obtained for both EBT-XD and EBT4 films at 180 Gy s -1 . • The EBT-XD response at the center of the 720 Gy s -1 proton beam was enhanced by up to 5%. • These findings can explain the controversial dose-rate dependence reported previously.