Purpose: Stereotactic body radiotherapy (SBRT) of the spine requires complex treatment planning to achieve high dose conformity, rapid dose falloff, and maximum dose constraints for the spinal cord. In this study, we aimed to (1) determine an appropriate planning technique for spine SBRT on the Varian Halcyon, (2) compare plan quality metrics using 4 and 6 arcs, and (3) quantify dose deviations due to rotational setup errors.
Methods: 15 patients from our institution were retrospectively planned on the Halcyon to a dose of 3,000 cGy using 4 and 6 arcs. For each case, the optimization objectives were adjusted manually by an experienced planner and the final plans were renormalized such that the PTV D90% = 100% to allow for the comparison of relevant dose metrics. Dose deviations due to rotational setup errors were investigated using 3DSlicer by simulating 1-degree rotations in the pitch, yaw, and roll directions and recalculating doses to relevant organs-at-risk (OAR).
Results: Plan coverage, conformity index (CI), and gradient measure (GM) were similar between the 4 and 6-arc plans with D95% = 94.9 ± 2.2% vs. 94.8 ± 2.2%, CI = 0.95 ± 0.03, and GM = 1.21 ± 0.29 vs 1.19 ± 0.25. Maximum doses to the spinal cord and thecal sac were reduced from 15.1 ± 2.6 Gy to 14.1 ± 2.5 Gy and 24.9 ± 2.9 Gy to 24.1 ± 3.2 Gy, respectively, using the 6-arc technique. With a 1-degree rotation, the spinal cord maximum dose deviated from -1.26% to 6.95%, corresponding to an absolute change of -0.16 Gy to 1.07 Gy.
Conclusion: SBRT treatment plans generated using 4 or 6 arcs on the Halcyon were comparable and showed significant improvements in plan quality over a previously studied 3-arc technique. Rotational setup errors result in minimal dose deviations when limited to = 1 degree.