Response characterization of EBT-XD radiochromic films in megavoltage photon and electron beams

Purpose: The purpose of this study was to investigate the beam quality dependence of the EBT-XD radiochromic films for megavoltage photon and electron beam irradiations by studying their spectral response. Dose, dose rate, and interbatch dependencies were investigated as well. Methods: EBT-XD and EBT3 radiochromic films were cut into 5 cm² × 5 cm² pieces, placed between solid water phantoms, and irradiated with 6 and 15 MV photon beams, 6 and 10 MV flattening filter free photon beams, and 6 and 20 MeV electron beams at dose levels between 0.5 and 50 Gy. In order to measure the spectral response, the films were illuminated by a deuterium and tungsten-halogen lamp and net absorption was measured in 400–800 nm spectral range using a fiber-coupled optical spectrometer. Film samples were then analyzed using a flatbed scanner in the red, green, and blue channel to obtain the optical density of the films. Results: The net absorption spectra of the EBT-XD and EBT3 films showed two absorption bands centered at 635 and 585 nm, characteristic of the EBT model. However, for a given dose, the EBT-XD films showed lower net absorbance compared to the EBT3 films. At a given dose level, the net absorption spectra and dose–response curves for EBT-XD films showed only slight variations across various beam qualities. When a calibration curve obtained from a given beam quality is used to measure dose from films irradiated with other beam qualities, the maximum uncertainty in the calculated dose, in ranges 5–40 and 8–50 Gy for red and green channels, respectively, were ~3.1% and ~2.4%. Conclusions: The response of the EBT-XD films is dose rate independent but batch dependent. No significant beam quality dependence was observed in EBT-XD films in the dose range up to 50 Gy.