Can intermediate-energy sources lead to elevated bone doses for prostate and head & neck high-dose-rate brachytherapy?

PURPOSE: Several radionuclides with high (Co-60, Se-75) and intermediate (Yb-169, Gd-153) energies have been investigated as alternatives to Ir-192 for high-dose-rate brachytherapy. The purpose of this study was to evaluate the impact of tissue heterogeneities for these five high- to intermediate-energy sources in prostate and head & neck brachytherapy. METHODS AND MATERIALS: Treatment plans were generated for a cohort of prostate (n = 10) and oral tongue (n = 10) patients. Dose calculations were performed using RapidBrachyMCTPS, an in-house Geant4-based Monte Carlo treatment planning system. Treatment plans were simulated using Co-60, Ir-192, Se-75, Yb-169, and Gd-153 as the active core of the microSelectron v2 source. Two dose calculation scenarios were presented: (1) dose to water in water (D-w,D-w), and (2) dose to medium in medium (D-m,D-m). RESULTS: D-w,D-w overestimates planning target volume coverage compared with D-m,D-m, regardless of photon energy. The average planning target volume D-90 reduction was similar to 1% for high-energy sources, whereas larger differences were observed for intermediate-energy sources (1%-2% for prostate and 4%-7% for oral tongue). Dose differences were not clinically relevant (<5%) for soft tissues in general. Going from D-w,D-w to D-m,D-m, bone doses were increased two- to three-fold for Yb-169 and four- to five-fold for Gd-153, whereas the ratio was close to similar to 1 for high-energy sources. CONCLUSIONS: D-w,D-w underestimates the dose to bones and, to a lesser extent, overestimates the dose to soft tissues for radionuclides with average energies lower than Ir-192. Further studies regarding bone toxicities are needed before intermediate-energy sources can be adopted in cases where bones are in close vicinity to the tumor. (C) 2019 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.