Robustness of patient positioning for interfractional error in carbon ion radiotherapy for stage I lung cancer: Bone matching versus tumor matching

Background and purpose: Patient positioning was compared by tumor matching (TM) and conventional bony structure matching (BM) in carbon ion radiotherapy for stage I non-small cell lung cancer to evaluate the robustness of TM and BM in determining interfractional error. Material and methods: Sixty irradiation fields were analyzed. Computed tomography (CT) images acquired before treatment initiation for confirmation (Conf-CT) were obtained under the same settings as the treatment planning CT images and used to evaluate both positioning methods. The dose distributions were recalculated for Conf-CT using both BM and TM, and the dose–volume histogram parameters [V_95% of clinical target volume, V_5Gy(RBE) of normal lung, and acceptance ratio (ratio of cases with V_95% > 95%)] were evaluated. The required margin, which in 90% of cases achieved the acceptable condition, was also examined. Results: Using BM and TM, the median V_95% was 98.93% and 100% (p < 0.001) and the mean V_5Gy(RBE) was 135.9 and 125.8 (p = 0.694), respectively. The estimated required margins were 7.9 and 3.3 mm and increased by 53.9% and 2.5% of V_5Gy(RBE), respectively, compared with planning. Conclusions: TM ensured a better dose distribution than did BM. To enable TM, volumetric imaging is crucial and should replace 2D radiographs for carbon therapy of stage I lung cancer.