Dose distribution estimation toward CT-less adaptive carbon ion radiotherapy for liver tumors using the divided-volume matching technique.

Objective &#xD;Dose distribution estimation during the treatment course is essential for carbon ion radiotherapy because beam ranges are highly sensitive to density changes along beam paths, triggering the adaptive re-planning at an appropriate time. This study aims to investigate the feasibility of evaluating daily dose distributions using the divided-volume matching (DVM) technique without additional daily computed tomography (CT) scans for adaptive carbon ion radiotherapy for liver tumors. &#xD;Approach&#xD;Phantom and patient data were included in this study. The developed in-house DVM software generated DVM CTs based on the existing resources, the planning CT, and orthogonal two-dimensional (2D) setup images. Bone matching (BM) and tumor matching (TM) are the two common ways of patient positioning correction to determine the isocenter for the irradiation of the day. We compared the dose distributions between DVM and in-room CTs with different isocenters based on BM or TM to verify whether the DVM CTs sufficiently represent the in-room CTs for daily dose distribution evaluations. &#xD;Main results&#xD;For the phantom study, the clinical target volume (CTV) coverage (V95%) differences between the in-room and the DVM CTs were <2%, and their dose distribution patterns were similar. For clinical data, the 3%/3 mm gamma passing rates were over 96%, and the planning target volume (PTV) coverage (V95%) differences were <3% between the in-room and DVM CTs in nine out of ten patients. With different isocenters, the dose coverage of the DVM CT changed consistently with those of the in-room CT. &#xD;Significance&#xD;The DVM technique enabled the evaluation of daily dose distributions without additional CT scans and was shown to be feasible in carbon ion radiotherapy for liver tumors.