Dosimetric response of Gafchromic™ EBT‐XD film to therapeutic protons

The EBT‐XD model of Gafchromic™ films has a broader optimal dynamic dose range, up to 40 Gy, compared with its predecessor models. This characteristic has made EBT‐XD films suitable for high‐dose applications, such as stereotactic body radiotherapy and stereotactic radiosurgery, as well as ultra‐high dose rate FLASH radiotherapy. The purpose of the current study was to characterize the dependence of EBT‐XD film response on linear energy transfer (LET) and dose rate of therapeutic protons from a synchrotron. A clinical spot‐scanning proton beam was used to study LET dependence at three dose‐averaged LET values of 1.0 keV/μm, 3.6 keV/μm, and 7.6 keV/μm. A research proton beamline was used to study dose rate dependence at 150 Gy/s in the FLASH mode and 0.3 Gy/s in the non‐FLASH mode. Film response data from dose‐averaged LET values of 0.9 keV/μm and 9.0 keV/μm of the proton FLASH beam were also compared. Film response data from a clinical 6‐MV photon beam were used as a reference. Both the gray value method and optical density (OD) method were used in film calibration. Calibration results using a specific OD calculation method and a generic OD calculation method were compared. The four‐parameter NIH Rodbard function and three‐parameter rational function were compared in fitting the calibration curves. Experimental results showed that the response of EBT‐XD film is proton LET dependent, but independent of dose rate. Goodness‐of‐fit analysis showed that using the NIH Rodbard function is superior for both protons and photons. Using the “specific OD + NIH Rodbard function” method for EBT‐XD film calibration is recommended. The results of dose‐rate independence of EBT‐XD film to protons are shown in Figure G1. The FLASH dose rate is 150 Gy/s and the non‐FLASH dose rate is 0.3 Gy/s. The linear energy transfer dependence of EBT‐XD film to proton beams is shown in Figure G2. Film calibration curves from three different dose‐averaged linear energy transfer values (1.0 keV/μm, 3.6 keV/μm, and 7.6 keV/μm) using clinical 87.2 MeV protons are compared in Figure G2(A). Film calibration curves from two dose‐averaged linear energy transfer values (0.9 keV/μm and 9.0 keV/μm) using FLASH 87.2 MeV protons are compared in Figure G2(B).