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

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 6–20 MeV 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 9 MeV electron beams while the difference was negligible for the higher energy electron beams. Beyond the buildup region (at a depth range of 22–26 mm for the 6 MeV beam and 38 mm for the 9 MeV 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 16 MeV electron beam, the dose discrepancy from water increases and reaches about 7.0 % at 70 mm depth for the lower halogen content PRESAGE^® formulations and 20 % at 66 mm depth for the original formulation. For the 20 MeV electron beam, the discrepancy drops to 6.0 % at 90 mm depth for the lower halogen content formulations and 18 % at 85 mm 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.