Dosimetric Considerations for Ytterbium-169, Selenium-75, and Iridium-192 Radioisotopes in High-Dose-Rate Endorectal Brachytherapy

Abstract

Purpose: To investigate differences between prescribed and postimplant calculated dose in ¹⁹²Ir high-dose-rate endorectal brachytherapy (HDR-EBT) by evaluating dose to clinical target volume (CTV) and organs at risk (OARs) calculated with a Monte Carlo-based dose calculation software, RapidBrachyMC. In addition, dose coverage, conformity, and homogeneity were compared among the radionuclides ¹⁹²Ir, ⁷⁵Se, and ¹⁶⁹Yb for use in HDR-EBT.

Methods and materials: Postimplant dosimetry was evaluated using 23 computed tomography (CT) images from patients treated with HDR-EBT using the ¹⁹²Ir microSelectron v2 (Elekta AB, Stockholm, Sweden) source and the Intracavitary Mold Applicator Set (Elekta AB, Stockholm, Sweden), which is a flexible applicator capable of fitting a tungsten rod for OAR shielding. Four tissue segmentation schemes were evaluated: (1) TG-43 formalism, (2) materials and nominal densities assigned to contours of foreign objects, (3) materials and nominal densities assigned to contoured organs in addition to foreign objects, and (4) materials specified as in (3) but with voxel mass densities derived from CT Hounsfield units. Clinical plans optimized for ¹⁹²Ir were used, with the results for ⁷⁵Se and ¹⁶⁹Yb normalized to the D₉₀ of the ¹⁹²Ir clinical plan.

Results: In comparison to segmentation scheme 4, TG-43-based dosimetry overestimates CTV D₉₀ by 6% (P = .00003), rectum D₅₀ by 24% (P = .00003), and pelvic bone D₅₀ by 5% (P = .00003) for ¹⁹²Ir. For ¹⁶⁹Yb, CTV D₉₀ is overestimated by 17% (P = .00003) and rectum D₅₀ by 39% (P = .00003), and pelvic bone D₅₀ is significantly underestimated by 27% (P = .007). Postimplant dosimetry calculations also showed that a ¹⁶⁹Yb source would give 20% (P = .00003) lower rectum V₆₀ and 17% (P = .00008) lower rectum D₅₀.

Conclusions: Ignoring high-Z materials in dose calculation contributes to inaccuracies that may lead to suboptimal dose optimization and disagreement between prescribed and calculated dose. This is especially important for low-energy radionuclides. Our results also show that with future magnetic resonance imaging-based treatment planning, loss of CT density data will only affect calculated dose in nonbone OARs by 2% or less and bone OARs by 13% or less across all sources if material composition and nominal mass densities are correctly assigned.