Monte Carlo assessment of a treatment planning system for intraoperative radiotherapy in the presence of tissue heterogeneities

BACKGROUND: Radiance is a Monte Carlo-based treatment planning software integrated into the Intrabeam system for 50-kVp X-rays intraoperative radiotherapy (IORT). PURPOSE: This study evaluates Radiance accuracy in calculating absorbed dose in IORT of the breast including the effect of tissue heterogeneities. METHODS: Dose distributions from IORT were calculated in 88 virtual phantoms of different tissue-equivalent materials. Dose was calculated independently using the penEasy code for the PENELOPE Monte Carlo engine and compared to Radiance. Depth dose curves (DDCs) were compared through gamma analysis using 5%/0.5 mm and 3%/0.5 mm criteria. RESULTS: When comparing Radiance vs penEasy all passing rates were above 95%. Notable differences from dose to water emerged for some tissues: cortical bone absorbed about four times more than water, while adipose tissue received about 45% less. These variations were affected by atomic number more than density. CONCLUSION: Radiance results show strong agreement with penEasy simulations. Furthermore, it was observed that IORT absorbed doses can fluctuate by ± 50% due to heterogeneities surrounding the applicator. This highlights that the prescribed dose and the absorbed dose in the applicator surface do not necessarily coincide in non-water tissues, emphasizing the critical need to account for specific tissue characteristics during treatment planning.