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

Abstract

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. 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. 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. 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.