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 <0.40 mm and 0.30 degrees, respectively, for all directions. For the respiratory motion patterns of 15 patients, the mean +/- SD of the RMSEs of the translation and rotation positional errors was <0.55 mm and 0.85 degrees, respectively, for all directions. The gamma(3%/2mm)values under translation with/without gating were 97.6 +/- 2.2%/80.9 +/- 18.1% and 96.8 +/- 2.3%/80.0 +/- 17.0% in the coronal and sagittal planes, respectively. Further, the gamma(3%/2mm)values under rotation with/without gating were 91.5 +/- 6.5%/72.8 +/- 18.6% and 90.3 +/- 6.1%/72.9 +/- 15.7% in the coronal and sagittal planes, respectively. Conclusions The developed 6DoF robotic phantom system could determine the translational and rotational motions of lung tumors with high accuracy. Further, respiratory-gating radiotherapy with real-time tumor monitoring using an internal surrogate marker was effective in compensating for the translational motion of lung tumors but not for correcting their rotational motion.