The relation between XR-QA2 and RT-QA2 GafchromicTM film optical density and absorbed dose in water produced by radionuclides
Authors: Joubert Maria M , van Eeden Déte , du Plessis Freek C P 
Published: February 2021

Abstract

Purpose. In this study, Monte Carlo (MC) simulations were done to relate the dose-response of the film to that in water. The effect of backscattering materials (PMMA, lead, polystyrene, and air) was investigated on its influence on film density for radionuclides including Am-241, Tc-99m, I-131, Cs-137. Methods. A BEAMnrc MC simulation was designed to score a phase-space file (PSF) below the container of the radionuclide under consideration to use as an input file for the subsequent DOSXYZnrc MC simulation. The geometry of the container holding the radionuclide was built using the component modules available in BEAMnrc. BEAMDP was used to investigate the container effect on the radionuclide spectrum as well as the fluence. The DOSXYZnrc simulation produced the absorbed dose in XR-QA2 and RT-QA2 GafchromicTM films. The DOSXYZnrc simulations were repeated for the GafchromicTM film now replaced with water to get the absorbed dose in water. From these results, conversion factors for the dose in water to the film dose for the different radionuclides, Am-241, Tc-99m, I-131, and Cs-137 were obtained. The actual film dose was calculated using the specific gamma exposure constant (Γ) at a distance of 50 cm for a point source approximation. From the BEAMnrc simulations, the particle fluence was extracted from PSFs to correct for the fluence at 0.1 cm below the sources from the fluence 50 cm away since the inverse square law will not apply to finite-size sources. The absorbed dose profiles in the film were compared to the absorbed dose profiles from the MC simulations. Results. A fitting function based on the neutron depletion model fits the optical density versus absorbed film dose data well and can be used as a calibration tool to obtain the film dose from its optical density. Lead as a backscatter material results in a higher optical density change but a lower absorbed dose. The XR-QA2 GafchromicTM film is more sensitive than the RT-QA2 GafchromicTM film, showing a more responsive optical density (OD) change in the energy range of radionuclides used in this study. Conversion factors were determined to convert the dose in water to the dose in GafchromicTM film. The Am-241 and I-131 simulated absorbed dose in the film to dose in water does not fluctuate as much as the simulated absorbed dose in film and water when using Tc-99m and Cs-137. Validation was shown for the comparison of the film and MC simulation absorbed dose profiles. Conclusions. MC BEAMnrc simulations are useful to simulate radionuclides and their containers. BEAMDP extracted energy spectra showed that the radionuclide containers produced a Compton effect on the energy spectra and added filtration on the lower spectral photon components. Extracted fluence ratios from PSFs were used to calculate the absorbed dose value at 0.1 cm distance from the source. By using the fit function, the dose in the film can be determined for known optical density values. The effect of the backscatter materials showed that the XR-QA2 GafchromicTM film results in higher optical density values than the RT-QA2 GafchromicTM film. The absorbed dose in both the films is comparable but not for a radionuclide such as Am-241 with an activity of 74MBq. The lead backscatter material showed to be the most prominent in optical density enhancement, and the air equivalent material was the least prominent. The XR-QA2 GafchromicTM film is the most sensitive and will be the best option if working with low energies. The absorbed dose in the XR-QA2 GafchromicTM film also showed a good comparison to the absorbed dose in water for the Am-241 radionuclide with an activity of 74MBq. The absorbed dose in the films compares well to the MC simulated doses.