Dosimetric Evaluation of CyberKnife Synchrony System for Liver Tumors With Respiratory Phase Shifts

Background/aim: This study evaluated the effects of the respiratory phase shifts between liver tumor and chest wall motions on the dose distribution for the CyberKnife Synchrony respiratory tracking system (SRTS).

Patients and methods: Eight patients who received stereotactic body radiotherapy for hepatocellular carcinoma or liver metastases were analyzed. Three-dimensional (3D) motion of the implanted fiducial markers and vertical motion of the sternal bone were derived from the four-dimensional computed tomography (4D-CT) images acquired with a 320-row area detector CT. For each patient, Gaussian random numbers were generated for the standard deviation of the tracking error calculated from the phase shift and a literature. For each voxel of the target, the dose delivered from each beam was calculated 100 times with the random 3D offsets representing the tracking error.

Results: The median respiratory phase shifts were 6.0% and 4.6% for the anterior-posterior (AP) and superior-inferior (SI) directions, respectively. The median motion tracking errors influenced by respiratory phase shifts were 1.21 mm and 0.96 mm for the AP and SI directions, respectively. The change in the dose covering 90% of the target (D_90%) was within 1.1% when median phase shifts were considered. When evaluating the 90^th percentile of the phase shifts, the D_90% decreased up to 6.6%.

Conclusion: We have developed a technique to estimate the impact of the respiratory phase shifts on the dose distribution of a liver tumor treated with the SRTS. The calculation of the respiratory phase shifts from the area-detector 4D-CT will be valuable to improve the tracking accuracy of the SRTS.