Evaluation of Patient-specific Quality Assurance for Spot-Scanning Carbon Ion Radiotherapy

[Purpose] In the QST hospital, carbon ion radiotherapy (CIRT) using spot-scanning was started in 2011. In order to check each individual CIRT treatment plan and its treatment delivery, the scanning system and its treatment planning system require dosimetric patient-specific Quality Assurance (PQA) before CIRT. We report results of PQA in half year of 2017.[Methods] For the PQA, all the planned beams are recalculated on a water phantom with treatment planning software. The recalculated dose distributions are compared with the measured distributions using a 2D ionization chamber array at three depth, including 95% area of prescription dose. These are evaluated using 3%/3 mm gamma criteria with a pass threshold of 90%. We analyzed PQA results for about 1500 shots from September 2017 to March 2018, evaluated the pass rate for 10 disease sites with various target volume, energy and measurement depth.[Results] The pass rate for prostate, head and neck, and bone and soft tissue were 95.9%, 99.3%, and 91.5%, respectively. 95.9% of lung plans with low energy beam passed. Overall, 95% of the measured beams for all shots passed the gamma criteria, and were consistent with the plan. Although the rest of 5% failed the criteria, there were no dependence of measurements depth and disease sites in these failures.[Conclusion] The results of the PQA in the second half of FY2017 was analyzed. The overall quality assurance measurement passing rate was 95% for the gamma index criteria of 3%/3 mm. It was obvious that there was no disease sites or target dependency in these pass rates. Therefore, we could ensure the safety of the CIRT treatment plans.第119回日本医学物理学会学術大