Assessing the robustness of dose distributions in carbon ion prostate radiotherapy using a fast dose evaluation system

Purpose: We developed a software program for swiftly calculating dose distributions for carbon ion beams. This study aims to evaluate the accuracy of dose calculations using this software and assess the robustness of dose distribution in treating prostate cancer.

Methods: At the Osaka Heavy Ion Therapy Center, markers are inserted into the prostate gland and used for position verification. To account for geometric changes along the beam path due to marker translation, a beam-specific planning target volume (bsPTV) is set for each beam. To validate the accuracy of the dose calculations using the developed software, dose distributions for prostate and sarcoma cases were calculated and compared with the treatment planning system. To assess the robustness of the dose distribution, position verification data from 346 cases were utilized to reproduce dose distributions for three matching methods: bone matching, widely adopted in most particle therapy centers; marker translation, which involves direct translation to markers without bone matching; and marker translation after bone matching. The coverage of the target (D₉₉ of clinical target volume (CTV)) was assessed to evaluate the robustness of the dose distribution. Additionally, statistical analyses were conducted for the dose distributions of each matching method.

Results: The dose calculation for a single condition can be completed very quickly. Statistical analysis revealed significant differences among dose distributions considering the three matching methods. When irradiation was performed with bone matching only, the D₉₉ was reduced by more than 10% in approximately 7.5% of cases, making it as the poorest among the three matching methods. However, there was no significant reduction in target coverage with the other two methods.

Conclusion: We have demonstrated the accuracy of the developed software for rapidly calculating dose distributions for carbon ion beams and confirmed the robustness of the dose distributions based on the bsPTV.