EBT3 film calibration in the Bragg peak region for proton beams below 5 MeV

Abstract

This work presents the study of the optical density as function of absorbed dose for radiochromic films (model EBT3 Gafchromic) irradiated with protons of energies below 5 MeV. Once the calibration curves are obtained, the study of the optical density as function of the linear energy transfer (LET) is presented. Such studies are useful in proton dosimetry applied to pre-clinical and in vitro radiobiological research.Measurements were performed at the National Centre of Accelerators (CNA) in Seville with a proton beam produced by a 3 MV Tandem accelerator. The incident energy onto the films was measured with an ion implanted silicon detector, and the particle fluence was monitored using a parallel plate ionization chamber. With this proton beam it is possible to investigate the interaction of high LET protons with EBT3 films with narrow energy spreads, as compared with therapeutic proton beams in the Bragg peak region. The data of the beam energy at the position of the film were used to simulate the energy deposition in the active layer of the film, by means of Monte Carlo codes. The measurements were performed for dose values up to 30 Gy and for five values of incident energy, obtained with a passive degradation of the beam by interposing mylar foils of different thickness upstream the EBT3 film.For each energy and corresponding value of proton LET, curves of the optical density as function of the dose are provided for the three color channels red, green and blue (RGB). The dose has been calculated considering two scenarios: dose to water (considering the film composition made by water) and dose to lucite (assuming the real composition of the film). The first corresponds to the case of clinical interest and the second to the real situation of the measurements. The comparison shows a non-negligible shift of the calculated doses even if density corrections are included. In the plateau region of the Bragg curve the film response can be well described by a simple polynomial function (at low doses up to about 4 MeV the response is almost linear). In regions covering the Bragg maximum the film response suffers a quenching and the optical density values were found to be 20–30 % lower than expected.