Optimization of collimator angles in dual-arc volumetric modulated arc therapy planning for whole-brain radiotherapy with hippocampus and inner ear sparing
Wuji Sun, Kunzhi Chen, Yu Li, Wenming Xia, Lihua Dong, Yinghua Shi, Chao Ge, Xu Yang, Libo Wang, Huidong Wang
Abstract
Abstract To optimize the collimator angles in dual-arc volumetric modulated arc therapy (VMAT) plans for whole-brain radiotherapy with hippocampus and inner ear sparing (HIS-WBRT). Two sets of dual-arc VMAT plans were generated for 13 small-cell lung cancer patients: (1) The collimator angles of arcs 1 and 2 ( θ 1 / θ 2 ) were 350°/10°, 350°/30°, 350°/45°, 350°/60°, and 350°/80°, i.e., the intersection angle of θ 1 and θ 2 (Δ θ ) increased. (2) θ 1 / θ 2 were 280°/10°, 300°/30°, 315°/45°, 330°/60°, and 350°/80°, i.e., Δ θ = 90°. The conformity index ( CI ), homogeneity index ( HI ), monitor units (MUs), and dosimetric parameters of organs-at-risk were analyzed. Quality assurance for Δ θ = 90° plans was performed. With Δ θ increasing towards 90°, a significant improvement was observed for most parameters. In 350°/80° plans compared with 350°/10° ones, CI and HI were improved by 1.1% and 25.2%, respectively; MUs were reduced by 16.2%; minimum, maximum, and mean doses ( D 100% , D max , and D mean , respectively) to the hippocampus were reduced by 5.5%, 6.3%, and 5.4%, respectively; D mean to the inner ear and eye were reduced by 0.7% and 5.1%, respectively. With Δ θ kept at 90°, the plan quality was not significantly affected by θ 1 / θ 2 combinations. The gamma-index passing rates in 280°/10° and 350°/80° plans were relatively lower compared with the other Δ θ = 90° plans. Δ θ showed a significant effect on dual-arc VMAT plans for HIS-WBRT. With Δ θ approaching 90°, the plan quality exhibited a nearly continuous improvement, whereas with Δ θ = 90°, the effect of θ 1 / θ 2 combination was insignificant.