Changes in scanning orientation effects of Gafchromic EBT-3 film irradiated with ultra-high dose rate proton beams

While the FLASH radiotherapy (FLASH RT) using ultra-high-dose-rate (UHDR) proton beams has the potential to achieve more effective cancer treatment, reliable and practical dosimetry of UHDR beams remains challenging. Although the commercial radiochromic films, named “Gafchromic film,” can be promising tools for the measurements of UHDR beam profiles, these films have some unfavorable features such as scanning orientation effects and lateral response artifacts. In this study, we examined the scanning orientation effects of Gafchromic EBT3 films irradiated with UHDR proton beams. The square sheets (5.5 × 5.5 cm2) of EBT3 film were irradiated with 8.7–8.8 Gy(H2O) at the plateau region of 139 MeV proton beams in the treatment room at the Nagoya Proton Therapy Center (NPTC) at three different dose rates: 0.29 Gy s−1, 170 Gy s−1, and 720 Gy s−1. The irradiated films were scanned in the reference and perpendicular orientations using two scanners: a flatbed scanner (FBS) (GT-X980, Seiko Epson Corp.) and an overhead scanner (OHS) (Aura, CZUR Tech Co., Ltd.). At the conventional dose rate (0.29 Gy s−1), the inverted red and green color profiles acquired by FBS in the perpendicular orientation were significantly lower than those in the reference orientation, as observed in previous studies using photon beams. Unexpectedly, such orientation effects were not observed in the FBS images of the EBT3 films irradiated with UHDR beams. The scanning orientation did not affect the color profiles of the OHS images at any dose rate tested. These results imply that the scanning orientation effect of the Gafchromic film is not a concern in beam profile measurements for FLASH RT. More comprehensive studies are needed to explain the mechanisms underlying these phenomena and to confirm the universality of the findings obtained in this study.