Optimum absorbed dose versus energy response of Gafchromic EBT2 and EBT3 films exposed to 20 160 kV x rays and 60Co gamma
High accuracy in the absorbed dose and small energy dependence are important features of a dosimeter to be considered ideal for medical applications. This work investigated the relation between the optimum absorbed dose in terms of combined standard uncertainty and energy response of EBT2 and EBT3 films exposed to five x ray beams and 60Co. A systematic analysis was performed to control the fitting procedure in order to find the parameter values that best describe the experimental data. Six EBT2 and three EBT3 film pieces per absorbed dose were exposed to 0.05 100 Gy and 0.1 15 Gy, respectively. HP Scanjet 7650 and Epson Expression 11000XL document scanners were used to read the EBT2 and EBT3 films, respectively both in transmission mode, 48 bit at 300 dpi spatial resolution. Images were analysed using ImageJ software. The results indicate that, independent of the photon energy beam, taking or not into account the uncertainty of the experimental data during the fitting process has a notable impact on the fit parameters for describing the measurements as well as on the uncertainty in the absorbed dose that can be measured with both films. Furthermore, the degree of energy dependence of both films depends on the absorbed dose and the minimum absorbed dose that can be measured with them is an intrinsic characteristic of each film model and depends on the photon energy as well as the colour channel. Lower the photon energy, smaller the combined standard uncertainty, possibly associated to higher ionisation density pattern of secondary electrons released locally within the film active layer by low photon energy comparing to high energy. Thus, based on the results of this work, we suggest the inclusion of the uncertainty on the experimental data in the fitting procedure of the film calibration curve and an optimum absorbed dose of 10 Gy for EBT3 in green channel and 3 Gy for EBT2 in red channel when use in modern radiotherapy techniques.