Optimal combination of collimator angles for dual-arc volumetric modulated arc therapy planning in stereotactic body radiotherapy for spinal metastases.

In planning the treatment of spinal metastases using stereotactic body radiotherapy (SBRT), the optimal blocking of the spinal cord to match the leaf travel can be achieved with a first-arc collimator angle of approximately 90°. We aim to clarify the optimal second-arc collimator angles when the first-arc collimator angle is fixed to 90° in dual-arc volumetric modulated arc therapy (VMAT). For this retrospective study, we considered 37 spinal segments with spinal metastases and created dual-arc VMAT plans. In the plans, 24 Gy in 2 fractions were prescribed, and the first-arc collimator angle was fixed to 90° while varying the second-arc collimator angle in increments of 15° from 0° to 90°. All the plans were normalized such that the planning organ-at-risk volume for the spinal cord D0.035 cc = 17 Gy and satisfied other dose constraints. D95% for the planning target volume (PTV), V100% for the overlap between the PTV and 10 mm expansion of the spinal cord, modified gradient index, monitor unit, and 3%/1 mm gamma passing rates were compared between different second-arc collimator angles using the Wilcoxon signed-rank test and Bonferroni correction. PTV D95% and overlap V100% were the highest for a second-arc collimator angle of 45° and decreased as the angle approached either 0° or 90°. The maximum mean differences of PTV D95% and overlap V100% were -2.66% (90° vs 45°, p < 0.0024) and -5.49% (90° vs 45°, p < 0.0024), respectively. Moreover, the second-arc collimator angle of 45° was the least suitable in terms of the modified gradient index. The required monitor unit increased from the second-arc collimator angle of 15° to 45°, and the 3%/1 mm gamma passing rates reached over 95% for the evaluated second-arc collimator angles of 15°, 30°, and 45°. We found that in the dual-arc VMAT plan for spine SBRT, second-arc collimator angles other than 90° were suitable, and 45° was the optimal angle in terms of target coverage including the area around the spinal cord.