||EXPERIMENTAL INVESTIGATION ON DOSIMETRIC CHARACTERIZATION OF GAFCHROMIC EBT3 AND EBT-XD FILMS FOR CLINICAL CARBON ION BEAM
米内, 俊祐 ,
新井, 千夏 ,
下山, 薫Nathalie, FOURNIER-BIDOZ
INTRODUCTION: Radiochromic film such as Gafchromic EBT3 and EBT-XD is very useful tool for 2-dimensional dosimetric measurements in radiotherapy, because it is a self-developing film and has very high spatial resolution. However, since it is known that its dose response depends on LET, considerable care has to be taken in ion beam radiotherapy. In this study, the dose responses of EBT3 and EBT-XD were experimentally investigated for clinical carbon ion beam at Heavy Ion Medical Accelerator in Chiba (HIMAC) and the relationship between the dose response and LET was shown.MATERIALS AND METHODS:The LET value of incident beam was changed with variable PMMA filters placed at the upstream of the film. Dose responses were investigated for 4 LET values between 12 and 169 keV/um and the LET dependency was investigated for additional 4 LET values. The absorbed dose and LET values were determined by measurements with a parallel-plate ionization chamber and Monte Carlo simulations with the PHITS code. The irradiated and unirradiated films were digitized 24 hour after irradiation with a commercial scanner, and pixel values of the red color channel were converted into a net optical density.RESULTS:Figure 1 shows measured dose response curves and its LET dependency of EBT3 shown as relative efficiency (RE) compared to 6-MV photon. The dose response of EBT-XD showed lower sensitivity compared to EBT3 as manufacturer specifications, but its tendency and the LET dependency were similar to EBT3. It was also confirmed that the repeatability was ~3% at a maximum when netOD is higher than 0.02, which was comparable to that for other radiation species.CONCLUSION:Dose responses of EBT3 and EBT-XD for clinical carbon ion beam were experimentally obtained. It was shown that EBT3 and EBT-XD can be applied to measurements of dose distributions by clinical carbon ion beam of which LET is known.