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Spectral analysis of the EBT3 radiochromic films for clinical photon and electron beams

PURPOSE
The purpose of this study was to investigate the spectral response of the EBT3 radiochromic films to different beam qualities for radiation therapy dosimetry. Dose, dose rate, and interbatch dependencies on the spectral response of the films were investigated as well.


METHODS
Pieces of EBT3 films placed between layers of solid water phantoms were irradiated with 6 and 15 MV photon beams, 6 and 10 MV-flattening filter free (FFF) photon beams, and 6 and 20 MeV electron beams at dose levels between 0.4 and 50 Gy. Net absorbance was measured as a function of wavelength from the spectra acquired in the wavelength range of 400-800 nm using a fiber-coupled spectrometer and broadband light source.


RESULTS
No significant change was observed in the absorption spectra of the EBT3 film from the same batch irradiated with the same amount of dose using different beam qualities. Also, no spectral change with dose rate was observed. The measured net absorbance per Gy was independent of beam quality in the 1-50 Gy dose range. Slight differences in the spectral shape and absorption band positions were observed in film samples from different batches. The net absorbance spectra showed two absorption bands centered around 634-636 nm (primary) and 583-585 nm (secondary). However, depending on the film batch, for doses above a certain level the primary absorption band appears to "split" into two bands centered around ~624-628 and ~641-645 nm.


CONCLUSIONS
The spectral shape of the EBT3 radiochromic films irradiated with photons (including FFF) and electron beams is beam quality and dose rate independent; however, it varies with dose level, batch, and spectroscopy system used.