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

PurposeSeveral radionuclides with high (⁶⁰Co, ⁷⁵Se) and intermediate (¹⁶⁹Yb, ¹⁵³Gd) energies have been investigated as alternatives to ¹⁹²Ir 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 MaterialsTreatment 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 ⁶⁰Co, ¹⁹²Ir, ⁷⁵Se, ¹⁶⁹Yb, and ¹⁵³Gd as the active core of the microSelectron v2 source. Two dose calculation scenarios were presented: (1) dose to water in water (D_w,w), and (2) dose to medium in medium (D_m,m).ResultsD_w,w overestimates planning target volume coverage compared with D_m,m, regardless of photon energy. The average planning target volume D₉₀ reduction was ～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,w to D_m,m, bone doses were increased two- to three-fold for ¹⁶⁹Yb and four- to five-fold for ¹⁵³Gd, whereas the ratio was close to ～1 for high-energy sources.ConclusionsD_w,w underestimates the dose to bones and, to a lesser extent, overestimates the dose to soft tissues for radionuclides with average energies lower than ¹⁹²Ir. 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.