The difference of scoring dose to water or tissues in Monte Carlo dose calculations for low energy brachytherapy photon sources

Purpose: The goal of this work is to compare D-m,D-m (radiation transported in medium; dose scored in medium) and D-w,D-m (radiation transported in medium; dose scored in water) obtained from Monte Carlo (MC) simulations for a subset of human tissues of interest in low energy photon brachytherapy. Using low dose rate seeds and an electronic brachytherapy source (EBS), the authors quantify the large cavity theory conversion factors required. The authors also assess whether applying large cavity theory utilizing the sources' initial photon spectra and average photon energy induces errors related to spatial spectral variations. First, ideal spherical geometries were investigated, followed by clinical brachytherapy LDR seed implants for breast and prostate cancer patients. Methods: Two types of dose calculations are performed with the GEANT4 MC code. (1) For several human tissues, dose profiles are obtained in spherical geometries centered on four types of low energy brachytherapy sources: I-125, Pd-103, and Cs-131 seeds, as well as an EBS operating at 50 kV. Ratios of D-w,D-m over D-m,D-m are evaluated in the 0-6 cm range. In addition to mean tissue composition, compositions corresponding to one standard deviation from the mean are also studied. (2) Four clinical breast (using Pd-103) and prostate (using I-125) brachytherapy seed implants are considered. MC dose calculations are performed based on postimplant CT scans using prostate and breast tissue compositions. PTV D-90 values are compared for D-w,D-m and D-m,D-m. Results: (1) Differences (D-w,D-m/D-m,D-m-1) of -3% to 70% are observed for the investigated tissues. For a given tissue, D-w,D-m/D-m,D-m is similar for all sources within 4% and does not vary more than 2% with distance due to very moderate spectral shifts. Variations of tissue composition about the assumed mean composition influence the conversion factors up to 38%. (2) The ratio of D-90(w,D-m) over D-90(m,D-m) for clinical implants matches D-w,D-m/D-m,D-m at 1 cm from the single point sources. Conclusions: Given the small variation with distance, using conversion factors based on the emitted photon spectrum (or its mean energy) of a given source introduces minimal error. The large differences observed between scoring schemes underline the need for guidelines on choice of media for dose reporting. Providing such guidelines is beyond the scope of this work.