Room: AAPM ePoster Library
Purpose: The purpose of this study was to investigate the response of EBT-XD radiochromic films to proton therapy beams.
Methods: EBT-XD radiochromic film samples from two different batches were placed between layers of solid water phantoms and irradiated with proton beam, with 1 Gy to 10 Gy dose, using a Mevion S250 passive scattering proton therapy system. For comparison, films samples were irradiated with the same dose using 6 MV photon beam produced by a clinical linear accelerator. For each film sample, the net absorbance spectrum was measured using an optical spectrometer as a function of wavelength in 400–800 nm range. The net optical density (OD) of the films were also measured using a flatbed scanner.
Results: The net absorbance spectrum of the EBT-XD films showed two absorption bands centered on 636 nm (primary) and 585 nm (secondary), in both proton- and photon-irradiated films. However, in the proton-irradiated films, the absorbance is ~5% lower than that of the photon-irradiated films. No significant energy dependence was noted in the film response to different proton beam energies. The percentage depth dose (PDD) measurement using film led to underestimation of the dose by ~8% in the spread-out Bragg peak region when compared to the measurement using an ionization chamber. The variations in net OD of ~1.5-4%, ~1-5%, and ~5-30% were noted for red, green, and blue channels, respectively from the flatbed scanner.
Conclusions: The absorption spectrum of proton-irradiated films shows lower net absorbance compared to the films from the same batch irradiated with megavoltage photons. Like EBT3 films, in the high linear energy transfer regions of the proton beam, this film model also suffers from the quenching, manifested as under-measurement of the dose.