Track and dose-average LET dependence of Gafchromic EBT3 and MD-V3 films exposed to low-energy photons

Abstract

AbstractGafchromic films are widely used in radiotherapy using photons, electrons and protons. Dosimetric characteristics of the films in terms of beam-quality is of great importance for a better evaluation of the absorbed-dose in the clinic. In proton-therapy, film’s response has been reported in terms of track-average, LΔ,T, or dose-average, LΔ,D, linear energy transfer (LET), concluding that LΔ,D is a more reliable parameter than LΔ,T. Nonetheless, in photon-beams, the film’s response is generally scrutinised in terms of photon-energy. This work aimed at investigating, the total (TEF) and secondary (SE) electron fluence produced in EBT3 and MD-V3 films exposed to 20 kV-160 kV x-ray and 60Co beams and their corresponding LΔ,T and LΔ,D to determine their influence on the film’s relative-efficiency, REFilm. Regardless the film-model, at energies below 100 keV, LΔ,D for TEF are about 1.7 to 2.5 times those of LΔ,T while for SE they are relatively similar (8–29%). For 60Co-gamma, LΔ,D for TEF and SE are approximately 9 and 4 times LΔ,T, respectively, which implies that LΔ,D is more important for high-photon energies. Independent of the electron-fluence and film-model, REFilm is almost constant at low average-LET, rapidly increases and thereafter steadily rises with average-LET. The REFilm−LET curve indicated that LΔ,D is more sensitive to small change than LΔ,T and if it is evaluated for SE, it would even be more appropriate to better describing the dosimeter response induced by photons in terms of ionization-density instead of LΔ,T for TEF, as generally done. Based on these results, once can conclude that the effect of the average-LET on the film’s response should be considered when use for clinical-dosimetry using photons and not only the energy.