Water equivalence evaluation of PRESAGE® formulations for megavoltage electron beams: a Monte Carlo study

Abstract

To investigate the radiological water equivalency of three different formulations of the radiochromic, polyurethane based dosimeter PRESAGE(®) for three dimensional (3D) dosimetry of electron beams. The EGSnrc/BEAMnrc Monte Carlo package was used to model 6-20MeV electron beams and calculate the corresponding doses delivered in the three different PRESAGE(®) formulations and water. The depth of 50% dose and practical range of electron beams were determined from the depth dose calculations and scaling factors were calculated for these electron beams. In the buildup region, a 1.0% difference in dose was found for all PRESAGE(®) formulations relative to water for 6 and 9MeV electron beams while the difference was negligible for the higher energy electron beams. Beyond the buildup region (at a depth range of 22-26mm for the 6MeV beam and 38mm for the 9MeV beam), the discrepancy from water was found to be 5.0% for the PRESAGE(®) formulations with lower halogen content than the original formulation, which was found to have a discrepancy of up to 14% relative to water. For a 16MeV electron beam, the dose discrepancy from water increases and reaches about 7.0% at 70mm depth for the lower halogen content PRESAGE(®) formulations and 20% at 66mm depth for the original formulation. For the 20MeV electron beam, the discrepancy drops to 6.0% at 90mm depth for the lower halogen content formulations and 18% at 85mm depth for the original formulation. For the lower halogen content PRESAGE(®), the depth of 50% dose and practical range of electrons differ from water by up to 3.0%, while the range of differences from water is between 6.5 and 8.0% for the original PRESAGE(®) formulation. The water equivalent depth scaling factor required for the original formulation of PRESAGE(®) was determined to be 1.07-1.08, which is larger than that determined for the lower halogen content formulations (1.03) over the entire beam energy range of electrons. All three of the PRESAGE(®) formulations studied require a depth scaling factor to convert depth in PRESAGE(®) to water equivalent depth for megavoltage electron beam dosimetry. Compared to the original PRESAGE(®) formulation, the lower halogen content formulations require a significantly smaller scaling factor and are thus recommended over the original PRESAGE(®) formulation for electron beam dosimetry.