Comparison of radiation dosimetry in water and in solid phantom materials for I-125 and Pd-103 brachytherapy sources: EGS4 Monte Carlo study.

A model based on the EGS4 Monte Carlo system is developed for calculating dose rate to water from an embedded low-energy brachytherapy source, given measurement data of dose rate to water within a water-substitute solid phantom for a source of given strength. The EGS4-based model is used to calculate point source dose rate distributions per unit source strength for water and for several species of solid phantoms. As a model for the dose rate distribution to an infinitesimal water inhomogeneity embedded in the solid phantom, the Monte Carlo is used to calculate dose rate to a thin spherical shell of water contained within the solid phantom at various distances centered on the source. Comparing this distribution to that calculated for a homogeneous water phantom, one can derive correction factors to determine dose rate to homogeneous water medium from measurements with water dose-calibrated detectors within the solid phantom. These factors are calculated for polymethylmethacrylate (PMMA or acrylic), solid water (WT1), and RW-1, a material optimized for low-energy dosimetry, with photon spectra from Pd-103 and from two commercial models of I-125 seed used as input. For model 6711 I-125 seeds at 1 cm in PMMA and WT1, the calculated ratios of dose rate to water to dose rate to water in the solid phantom are 0.893 and 1.038, respectively. Applying these factors to published, apparently divergent measurements of the specific dose rate constant based on measurements in PMMA and WT1 phantoms, the specific dose rate constant inferred for the model 6711 seed are found to be in excellent agreement: 0.878 cGy h-1U-1 (1.115 cGy- cm2h-1mCi-1) from the mean of the WT1 measurements and 0.879 cGy h-1U-1 from the PMMA measurements. Among the phantoms studied for Pd-103 and I-125, measurements in RW-1 would most closely approximate those for water.