Investigation of radiological properties and water equivalency of PRESAGE® dosimeters

Abstract

Purpose: PRESAGE® is a dosimeter made of polyurethane, which is suitable for 3D dosimetry in modern radiation treatment techniques. Since an ideal dosimeter is radiologically water equivalent, the authors investigated water equivalency and the radiological properties of three different PRESAGE® formulations that differ primarily in their elemental compositions. Two of the formulations are new and have lower halogen content than the original formulation. Methods: The radiological water equivalence was assessed by comparing the densities, interaction probabilities, and radiation dosimetry properties of the three different PRESAGE® formulations to the corresponding values for water. The relative depth doses were calculated using Monte Carlo methods for 50, 100, 200, and 350 kVp and 6 MV x-ray beams. Results: The mass densities of the three PRESAGE® formulations varied from 5.3% higher than that of water to as much as 10% higher than that of water for the original formulation. The probability of photoelectric absorption in the three different PRESAGE® formulations varied from 2.2 times greater than that of water for the new formulations to 3.5 times greater than that of water for the original formulation. The mass attenuation coefficient for the three formulations is 12%–50% higher than the value for water. These differences occur over an energy range (10-100 keV) in which the photoelectric effect is the dominant interaction. The collision mass stopping powers of the relatively lower halogen-containing PRESAGE® formulations also exhibit marginally better water equivalency than the original higher halogen-containing PRESAGE® formulation. Furthermore, the depth dose curves for the lower halogen-containing PRESAGE® formulations are slightly closer to that of water for a 6 MV beam. In the kilovoltage energy range, the depth dose curves for the lower halogen-containing PRESAGE® formulations are in better agreement with water than the original PRESAGE® formulation. Conclusions: Based on the results of this study, the new PRESAGE® formulations with lower halogen content are more radiologically water equivalent overall than the original formulation. This indicates that the new PRESAGE® formulations are better suited to clinical applications and are more accurate dosimeters and phantoms than the original PRESAGE® formulation. While correction factors are still needed to convert the dose measured by the dosimeter to an absorbed dose in water in the kilovoltage energy range, these correction factors are considerably smaller for the new PRESAGE® formulations compared to the original PRESAGE® and the existing polymer gel dosimeters.