Monte Carlo calculated absorbed-dose energy dependence of EBT3 and EBT4 films for 5–200 MeV electrons and 100 keV–15 MeV photons

Abstract

Purpose

To use Monte Carlo simulations to study the absorbed-dose energy dependence of GAFChromic EBT3 and EBT4 films for 5–200 MeV electron beams and 100 keV–15 MeV photon beams considering two film compositions: a previous EBT3 composition (Bekerat et al.) and the final composition of EBT3/current composition of EBT4 (Palmer et al.).

Methods

A water phantom was simulated with films at 5–50 mm depth in 5 mm intervals. The water phantom was irradiated with flat, monoenergetic 5–200 MeV electron beams and 100 and 150 keV kilovoltage and 1–15 MeV megavoltage photon beams and the dose to the active layer of the films was scored. Simulations were rerun with the films defined as water to compare the absorbed-dose response of film to water, $f^{-1}\left(Q\right)=\frac{D_{film}}{D_{water}}$.

Results

For electrons, the Bekerat et al. composition had variations in $f^{-1}\left(Q\right)$ of up to $(1.9\pm 0.1)\%$ from 5 to 200 MeV. Similarly, the Palmer et al. composition had differences in $f^{-1}\left(Q\right)$ up to $(2.5\pm 0.2)\%$ from 5 to 200 MeV. For photons, $f^{-1}\left(Q\right)$ varied up to $(2.4\pm 0.3)\%$ and $(4.5\pm 0.7)\%$ from 100 keV to 15 MeV for the Bekerat et al. and Palmer et al. compositions, respectively. The depth of films did not appear to significantly affect $f^{-1}\left(Q\right)$ for photons at any energy and for electrons at energies $>$ 50 MeV. However, for 5 and 10 MeV electrons, decreases of up to $(10.2\pm 1.1)\%$ in $f^{-1}\left(Q\right)$ were seen due to stacked films and increased beam attenuation in films compared to water.

Conclusions

The up to $(2.5\pm 0.2)\%$ and $(4.5\pm 0.7)\%$ variations in $f^{-1}\left(Q\right)$ for electrons and photons, respectively, across the energies considered in this study indicate the importance of calibrating films with the energy intended for measurement. Additionally, this work emphasizes potential issues with stacking films to measure depth dose curves, particularly for electron beams with energies $\le$10 MeV.