Spectroscopic analysis of irradiated radiochromic EBT-XD films in proton and photon beams

The response of GafchromicTM EBT-XD films to proton irradiation using a Mevion S250TM proton therapy system is studied in this work. Film samples from different batches were irradiated with doses from 0.25 Gy to 20 Gy using a Mevion S250 spread-out Bragg peak (SOBP) clinical proton beam. Using a fiber-coupled optical spectrometer, transmission spectra of the films were acquired over 400-800 nm range (with 2.5 nm resolution). The optical density (OD) of the films was also measured with a flatbed scanner. The characteristics of the film were investigated for dependencies on dose, inter-batch, energy, linear energy transfer (LET), and radiation type. Post-irradiation temporal growth of the OD of the films was also investigated. The net absorbance spectra of EBT-XD films exhibited two absorption peaks located at 636 nm and 585 nm. The measurement of percentage depth dose with the films showed ∼1%-5% and 7%-8% under-estimation on the dose in mid-SOBP and distal SOBP regions, respectively, compared to ionization chamber measurement. No significant energy dependence was noted in the OD of the films when irradiated at the same dose using different proton energies. Approximately 4%-10% drop in spectral response was seen for the proton-irradiated films relative to the films irradiated in photon beams. Temporal growth of the optical density showed a different developing slope between photon and proton irradiation in the first 12 h, but both reached a stable response 24 h after irradiation.