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 ⁶⁰Co and ¹⁹²Ir 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 (Dw,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 ¹⁹²Ir. 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 ⁶⁰Co and ¹⁹²Ir, with a greater dispersion for ¹⁹²Ir. Regarding the organs at risk (OARs), median percentage differences for breast V50% were 3% (5%) for ⁶⁰Co (¹⁹²Ir). Differences in median skin D2cc were found comparable, with a larger dispersion for ¹⁹²Ir, and the same applied to the lung D10cc and the aorta D2cc. TG-43 overestimates D2cc for the rectum and the sigmoid, with median differences from MC within 2% and a greater dispersion for ¹⁹²Ir. For the bladder, the median of the difference is greater for ⁶⁰Co (~2%) than for ¹⁹²Ir (~0.75%), demonstrating however a greater dispersion again for ¹⁹²Ir.

Conclusions
The magnitude of differences observed between TG-43 based and MC dosimetry and their smaller dispersion relative to ¹⁹²Ir, suggest that ⁶⁰Co HDR sources are more amenable to the TG-43 assumptions in clinical treatment planning dosimetry.