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 (θ1/θ2) were 350°/10°, 350°/30°, 350°/45°, 350°/60°, and 350°/80°, i.e., the intersection angle of θ1 and θ2 (Δθ) increased. (2) θ1/θ2 were 280°/10°, 300°/30°, 315°/45°, 330°/60°, and 350°/80°, i.e., Δθ = 90°. The conformity index (CI), homogeneity index (HI), monitor units (MUs), and dosimetric parameters of organs-at-risk were analyzed. Quality assurance for Δθ = 90° plans was performed. With Δθ 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 (D100%, Dmax, and Dmean, respectively) to the hippocampus were reduced by 5.5%, 6.3%, and 5.4%, respectively; Dmean to the inner ear and eye were reduced by 0.7% and 5.1%, respectively. With Δθ kept at 90°, the plan quality was not significantly affected by θ1/θ2 combinations. The gamma-index passing rates in 280°/10° and 350°/80° plans were relatively lower compared with the other Δθ = 90° plans. Δθ showed a significant effect on dual-arc VMAT plans for HIS-WBRT. With Δθ approaching 90°, the plan quality exhibited a nearly continuous improvement, whereas with Δθ = 90°, the effect of θ1/θ2 combination was insignificant.
Highlights
Whole-brain radiotherapy (WBRT) has been used for the treatment of brain metastases or the prophylactic cranial irradiation of patients with small-cell lung cancer in the past decades[1,2,3]
By rotating the collimator between arcs, a better dose distribution could be achieved with the multi-leaf collimator (MLC) moving directions orthogonal to each other[24]
With the optimal intersection angle, the specific collimator setting of each arc was investigated on the basis of plan quality evaluation and quality assurance (QA)
Summary
Whole-brain radiotherapy (WBRT) has been used for the treatment of brain metastases or the prophylactic cranial irradiation of patients with small-cell lung cancer in the past decades[1,2,3]. Current VMAT implementation does not allow collimator rotation during delivery, and the collimator angle in each arc is required to be determined before plan o ptimization[12]. For the complex-shaped target volume in HIS-WBRT, an optimal setting for the collimator angles must be applied, which, to the knowledge of the authors, has yet to be investigated. For dual-arc VMAT, the intersection angle between two collimator settings may have a significant effect on the plan optimization. The collimator angle of dual-arc VMAT for HIS-WBRT was optimized in two steps. With the optimal intersection angle, the specific collimator setting of each arc was investigated on the basis of plan quality evaluation and quality assurance (QA)
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