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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.

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