International journal of radiation oncology, biology, physics | 2019
Dosimetric Considerations for Ytterbium-169, Selenium-75 and Iridium-192 Radioisotopes in High Dose Rate Endorectal Brachytherapy.
Abstract
PURPOSE\nQuantification of impact in choices of dosimetric calculation method and radiation source for high dose rate endorectal brachytherapy (HDR-EBT) by evaluating dosimetric indices in the clinical target volume (CTV) and organs at risk (OARs). In addition, dose coverage, conformity and homogeneity were compared between the radionuclides 192Ir, 75Se and 169Yb for use in HDR-EBT.\n\n\nMETHODS\nMonte Carlo post-implant dosimetry using 23 computed tomography (CT) images from patients treated with HDR-EBT utilising 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 OARs 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 (3) but 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 segmentation scheme 1 D90 of the 192Ir clinical plan.\n\n\nRESULTS\nIn comparison to segmentation scheme 4, TG-43 based dosimetry overestimates CTV D90 by 6% (p=0.00003), rectum D50 by 24% (p=0.00003) and pelvic bone D50 by 5% (p=0.00003) for 192Ir. 75Se has generally similar dosimetric outcomes to 192Ir. For 169Yb, CTV D90 is overestimated by 17% (p=0.00003), rectum D50 by 39% (p=0.00003) and pelvic bone D50 is underestimated by 27% (p=0.007). Postimplant dosimetry calculations also showed that a 169Yb source, would give 20% (p=0.00003) lower rectum V60, and 17% (p=0.00008) lower rectum D50.\n\n\nCONCLUSIONS\nIgnoring high-Z materials in dose calculation contributes to inaccuracies which may lead to sub-optimal dose optimization and disagreement between prescribed and calculated dose. This is especially important for low energy radionuclides. Our results also show that with future MRI-based treatment planning, loss of CT-density data will affect calculated dose in non-bone OARs 2% or less, and bone OARs 13% or less across all evaluated radiation sources if material composition and nominal mass densities are correctly assigned.