Analysis of dosimetric impact of intrafraction translation and rotation during respiratory‐gated stereotactic body radiotherapy with real‐time tumor monitoring of the lung using a novel six degrees‐of‐freedom robotic moving phantom

Purpose This study aimed to develop a six degrees‐of‐freedom (6DoF) robotic moving phantom for evaluating the dosimetric impact of intrafraction rotation during respiratory‐gated radiotherapy with real‐time tumor monitoring in the lung. Materials and Methods Fifteen patients who had undergone respiratory‐gated stereotactic body radiotherapy (SBRT) with the SyncTraX system for lung tumors were enrolled in this study. A water‐equivalent phantom (WEP) was set at the tip of the robotic arm. A log file that recorded the three‐dimensional positions of three fiducial markers implanted near the lung tumor was used as the input to the 6DoF robotic moving phantom. Respiratory‐gated radiotherapy was performed for the WEP, which was driven using translational and rotational motions of the lung tumor. The accuracy of the 6DoF robotic moving phantom was calculated as the difference between the actual and the measured positions. To evaluate the dosimetric impact of intrafraction rotation, the absolute dose distributions under conditions involving gating and movement were compared with those under static conditions. Results For the sinusoidal patterns, the mean ± standard deviation (SD) of the root mean square errors (RMSEs) of the translation and rotation positional errors was