Microdosimetric modeling of the sensitometric curve of GafChromic films in the photon fields

The sensitometric curve of EBT3 GafChromic film located at a depth 5 cm of RW3 water-equivalent phantom exposed to 6 MV X-rays is investigated. Variation of optical density for absorbed doses less than 2 Gy is determined by the experimental measurement together with the microdosimetric one-hit detector model. It is found that this model needs two fitting parameters, a maximum optical density and a saturation parameter. Both of them depend on the film structure as well as the photon spectrum. Meanwhile, the saturation parameter is a function of the microdosimetric single-event distribution of specific energy, i.e., f₁(z). To calculate this distribution, irradiation of the films is simulated by the Geant4 toolkit. A sample of EBT3 film with 2 mm × 2 mm area is simulated. Active layer of the film is considered to contain 5000 cylindrical sensitive targets (SV) with 9.4 μm length and 1.62 μm diameter, located in random positions with different axial directions. The results obtained show that below an absorbed dose 1 Gy the maximum difference between the measured and the calculated optical densities is about 11%, while for the doses above 1 Gy the discrepancy is at most 3%. Eventually, it can be concluded that the sensitometric curve of EBT3 GafChromic film can satisfactorily be determined by the microdosimetric one-hit detector model, especially in the dose range above 1 Gy.