A comparative assessment of inhomogeneity and finite patient dimension effects in 60Co and 192Ir high-dose-rate brachytherapy

Purpose: To perform a comparative study of heterogeneities and finite patient dimension effects in Co-60 and Ir-192 high-dose-rate (HDR) brachytherapy. Material and methods: Clinically equivalent plans were prepared for 19 cases (8 breast, 5 esophagus, 6 gynecologic) using the Ir2.A85-2 and the Co0.A86 HDR sources, with a TG-43 based treatment planning system (TPS). Phase space files were obtained for the two source designs using MCNP6, and validated through comparison to a single source dosimetry results in the literature. Dose to water, taking into account the patient specific anatomy and materials (D-w,D-m), was calculated for all plans using MCNP6, with input files prepared using the BrachyGuide software tool to analyze information from DICOM RT plan exports. Results: A general TG-43 dose overestimation was observed, except for the lungs, with a greater magnitude for Ir-192. The distribution of percentage differences between TG-43 and Monte Carlo (MC) in dose volume histogram (DVH) indices for the planning target volume (PTV) presented small median values (about 2%) for both Co-60 and Ir-192, with a greater dispersion for Ir-192. Regarding the organs at risk (OARs), median percentage differences for breast V-50% were 3% (5%) for Co-60 (Ir-192). Differences in median skin D-2cc were found comparable, with a larger dispersion for Ir-192, and the same applied to the lung D-10cc and the aorta D-2cc. TG-43 overestimates D-2cc for the rectum and the sigmoid, with median differences from MC within 2% and a greater dispersion for Ir-192. For the bladder, the median of the difference is greater for Co-60 (similar to 2%) than for Ir-192 (similar to 0.75%), demonstrating however a greater dispersion again for Ir-192. Conclusions: The magnitude of differences observed between TG-43 based and MC dosimetry and their smaller dispersion relative to Ir-192, suggest that Co-60 HDR sources are more amenable to the TG-43 assumptions in clinical treatment planning dosimetry.