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

Gafchromic 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, L(Δ,T), or dose-average, L(Δ,D), linear energy transfer (LET), concluding that L(Δ,D) is a more reliable parameter than L(Δ,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 (60)Co beams and their corresponding L(Δ,T) and L(Δ,D) to determine their influence on the film’s relative-efficiency, RE(Film). Regardless the film-model, at energies below 100 keV, L(Δ,D) for TEF are about 1.7 to 2.5 times those of L(Δ,T) while for SE they are relatively similar (8–29%). For (60)Co-gamma, L(Δ,D) for TEF and SE are approximately 9 and 4 times L(Δ,T), respectively, which implies that L(Δ,D) is more important for high-photon energies. Independent of the electron-fluence and film-model, RE(Film) is almost constant at low average-LET, rapidly increases and thereafter steadily rises with average-LET. The RE(Film)−LET curve indicated that L(Δ,D) is more sensitive to small change than L(Δ,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 L(Δ,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.