On the dosimetric effect of heterogeneities and finite patient dimensions on 60Co HDR brachytherapy

Introduction Model based dose calculation algorithms (MBDCAs) for 192 Ir HDR brachytherapy have become clinically available to account for effects disregarded by TG-43 dosimetry (tissue heterogeneity and patient specific scatter conditions). Purpose To assess the relative benefit from introducing MBDCAs in Co-60 and Ir-192 HDR brachytherapy. Materials and methods Dual plans were prepared for 15 cases (gynaecological, esophagus, breast) using the Ir2.A85-2 and the Co0.A86 HDR sources with a TG-43 based TPS (SagiPlan®, Eckert&Ziegler BEBIG, Germany). Monte Carlo (MC) simulation dosimetry was performed for all plans using the MCNP6 code with input files prepared using the BrachyGuide software tool to parse information from DICOM RT exported plans. Differences between patient-specific (MC) and TG-43 dosimetry were compared in the form of 3D distributions and clinical plan indices and checked for statistical and clinical significance. Results were compared between Co-60 and Ir-192. Results Patient-specific dose distributions are clinically equivalent for the two isotopes. The effect of tissue heterogeneities and patient specific scatter conditions is less for Co-60 HDR treatments. A lower dose to critical organs close to the target and a small increase in the overdose volume were observed for Co-60. The choice of isotope was not found to have an impact on the prescribed dose. Conclusion Co-60 is equivalent to Ir-192 HDR brachytherapy, with similar target coverage. Besides logistical advantages owing to its longer half-life, Co-60 demonstrates reduced effects of tissue heterogeneities and patient dimensions that obviate the need for using MBDCAs. Disclosure Research supported in part by Eckert & Ziegler BEBIG.