Dosimetric comparison of HDR Co-60 and Ir-192 brachytherapy sources by model-based dose calculation algorithms and
Raghavendra Hajare1, Rohit Avinash Vadgaonkar1, Taushiful Hoque1
1Department of Radiation Oncology, Homi Bhabha Cancer Hospital and Research Centre, Tata Memorial Centre, Visakhapatnam, Andhra Pradesh, India.
Introduction:
Numerous dosimetric studies have reported clinically insignificant differences between iridium-192 and cobalt-60. However, such comparison studies were conducted using TG-43 algorithms where heterogeneity is not accounted. Model based dose calculation algorithms (MBDCA) significantly improve dosimetric accuracy for iridium-192, as validated in phantom and clinical dosimetric studies. Comparative dosimetry data for head and neck (HN) cancers and soft tissue sarcomas (STS) is scarce and mostly based on Monte Carlo simulations. This study evaluates how radionuclide choice and tissue heterogeneities affect multiple tumor sites in a clinical setting.
Material And Methods:
Seventy-five patients treated using TG-43 generated cobalt-60 plans were selected. For all patients, alternative equivalent plans were generated for cobalt-60 with TG-186 and for iridium-192 with both TG-43 and TG-186 algorithms.
Results:
For gynecological intracavitary brachytherapy (ICBT) and combined intracavitary-interstitial brachytherapy (IC-IS BT) plans, cobalt-60 delivered up to 4.9% higher doses to organs at risk (OARs) compared to iridium-192. For pure interstitial brachytherapy (ISBT) plans iridium-192 delivered upto 5.6% higher doses to OARs compared to cobalt-60, contrary to what was observed for ICBT/IC-IS BT plans. For breast, cobalt-60 delivered higher skin doses (+2.7%) and lower rib doses (-2.8%) when using TG-43; this difference in OAR doses reduced when using TG-186. For HN and STS, OAR doses were higher upto 21.9% with iridium-192 and these differences were exacerbated when using TG-186.
Conclusion:
The dosimetric differences between radionuclides are reduced when calculated with TG-186, suggesting that the effect of tissue heterogeneity might be higher for cobalt-60 compared to iridium-192. This dosimetric study highlights that TG-186 predicts the actual doses more accurately.
