Calcifications in low-dose rate prostate seed brachytherapy treatment: Post-planning dosimetry and predictive factors

Background and purpose: The brachytherapy dose algorithm of the American Association of Physicists in Medicine Task Group (TG) Report 43 overrides all tissue materials with water. In reality, dose discrepancies will occur around tissue calcifications. This study investigates these perturbations in lbw dose rate prostate brachytherapy dosimetry. Materials and methods: 43 cancer patients with prostatic calcifications are identified. Geant4 Monte Carlo (MC) simulations are made with materials assigned based on TG186 recommendations. Five dose calculation scenarios are presented: MC in water (MCW), MCW with calcifications, (MCCA), MCCA with seeds (MCCA(SEED)) and full tissue definition and seeds with dose to medium in medium (FMC) and dose to water in medium (FMC-D-w,D-m). Results: The Mean FMC prostate D90 (V100) difference relative to TG43 is -6.4% (range [-1.8, -14.1]) (-2.6% [-0.3, -6.7]). For MCCA we obtained -3.9% [-1.0, -8.7] (-1.5% [-0.2, -4.1]). The mean urethra D10 difference is -4.5% [-1.3, -9.9] for FMC, -2.4% [-0.7, -5.1] with MCCA. FMC-D-w,D-m D90 has a -0.45% smaller dose difference than FMC on average. The calcification/prostate volume ratio is a good predictor of dose perturbation (R-2 = 0.75). Conclusion: Based on these results, calcifications alter the dose coverage and may have severe dose perturbation that requires recalculation.