On the spectral characterization of radiochromic films irradiated with clinical proton beams.

Radiochromic films have been widely studied for clinical dosimetry in conventional external beam radiation therapy. With an increase in practice of proton therapy, such films are being conveniently used; however, their spectroscopic characterization for this modality is lacking. This work investigated the response of the EBT3 radiochromic films irradiated in a Mevion S250 clinical proton beam. Dose, dose rate, inter-batch, sensitivity, and linear energy transfer (LET) dependencies of the films were studied. Pieces of the radiochromic films from different batches were irradiated using a spread-out Bragg peak (SOBP) proton beam at dose levels between 0.1-15 Gy. Absorption spectra were measured in the wavelength range of 400-800 nm with 2.5 nm resolution. For comparison, the optical density of the films was measured using a flatbed scanner. The net absorbance spectra showed two characteristic absorption bands centered at 636 nm and 585 nm. However, a saturation effect, manifested as broadening/splitting appearance, was observed in the 636 nm band for doses beyond a certain batch-dependent level ~4-10 Gy, in the three different film batches studied. The differences in the spectral shape led to dose-response curves with variable sensitivity. In general a high spectral sensitivity was observed in 0.1-6 Gy range for the three film batches. For a given dose, no significant change in the spectra was observed with change in the dose rate. No significant dependency on the LET was observed for the EBT3 films irradiated with proton beams with dose-averaged LETs ranging from 1.14-6.50 keV µm studied in this work. However, at a given dose, ~5% lower spectral response was observed in the films irradiated with protons compared to their counterparts irradiated with photon beams.