Evaluation of Dose Robustness to SPR Variations for Different Beam Orientations In Carbon-Ion Radiotherapy for Pancreatic Cancer
Abstract
Purpose
To evaluate the effect of beam orientation on dose distribution robustness in carbon-ion radiotherapy for pancreatic cancer, focusing on stopping power ratio (SPR) variations and their impact on delivered dose accuracy.
Methods
A total of 90 treatment beams used for carbon-ion radiotherapy of pancreatic cancer between February 2022 and January 2025 were analyzed. The dataset included head-first supine vertical beams (virtual gantry angle [GA] = 0°, n = 30), head-first prone vertical beams (GA = 180°, n = 30), and head-first prone horizontal beams (GA = 270°, n = 30). The SPR differences within patients were calculated by comparing the planning CT with CT images acquired during the treatment course. The dose distribution calculated from the planning CT was used as the reference, and dose distributions during treatment were recalculated accordingly. Gamma analysis was then performed between the planned and the recalculated dose distributions. The Pearson’s correlation coefficients (CCs) between the mean SPR difference within the 20% isodose region and the gamma pass rate (GPR) with the criteria of 3%/2mm were evaluated for each beam orientation.
Results
The mean GPRs for GA = 0°, 180°, and 270° were 86.8%, 96.0%, and 91.0%, respectively, with corresponding CCs of -0.790, -0.337, and -0.828. A negative correlation was found between the mean SPR difference and the GPR, indicating reduced agreement between dose distributions with increasing SPR variation. The primary contributors to SPR variation were changes in patient body contour and positional variations of gastrointestinal gas, both of which strongly depended on beam orientation.
Conclusion
The robustness of dose distributions with respect to SPR variation and beam orientation was quantitatively evaluated. These results suggest that both beam arrangement and dose delivery accuracy during treatment should be carefully managed in carbon-ion radiotherapy for pancreatic cancer.