Noncoplanar VMAT for Brain Metastases: A Plan Quality and Delivery Efficiency Comparison With Coplanar VMAT, IMRT, and CyberKnife

Abstract

Purpose: To compare plan quality and delivery efficiency of noncoplanar volumetric modulated arc therapy with coplanar volumetric modulated arc therapy, intensity-modulated radiation therapy, and CyberKnife for multiple brain metastases. Methods: For 15 patients with multiple brain metastases, noncoplanar volumetric modulated arc therapy, coplanar volumetric modulated arc therapy, intensity-modulated radiation therapy, and CyberKnife plans with a prescription dose of 30 Gy in 3 fractions were generated. Noncoplanar volumetric modulated arc therapy and coplanar volumetric modulated arc therapy plans consisted of 4 noncoplanar arcs and 2 full coplanar arcs, respectively. Intensity-modulated radiation therapy plans consisted of 7 coplanar fields. CyberKnife plans used skull tracking to ensure accurate position. All plans were generated to cover 95% target volume with prescription dose. Gradient index, conformity index, normal brain tissue volume (V3Gy V24Gy), monitor units, and beam on time were evaluated. Results: Gradient index was the lowest for CyberKnife (3.49 + 0.65), followed by noncoplanar volumetric modulated arc therapy (4.21 + 1.38), coplanar volumetric modulated arc therapy (4.87 + 1.35), and intensity-modulated radiation therapy (5.36 + 1.98). Conformity index was the largest for noncoplanar volumetric modulated arc therapy (0.87 + 0.03), followed by coplanar volumetric modulated arc therapy (0.86 + 0.04), CyberKnife (0.86 + 0.07), and intensity-modulated radiation therapy (0.85 + 0.05). Normal brain tissue volume at high-to-moderate dose spreads (V24Gy V9Gy) was significantly reduced in non-coplanar volumetric modulated arc therapy over that of intensity-modulated radiation therapy and coplanar volumetric modulated arc therapy. Normal brain tissue volume for noncoplanar volumetric modulated arc therapy was comparable with noncoplanar volumetric modulated arc therapy at high-dose level (V24Gy V15Gy) and larger than CyberKnife at moderate-to-low dose level (V12Gy V3Gy). Monitor units was highest for CyberKnife (28 733.59 + 7197.85), followed by intensity-modulated radiation therapy (4128.40 + 1185.38), noncoplanar volumetric modulated arc therapy (3105.20 + 371.23), and coplanar volumetric modulated arc therapy (2997.27 + 446.84). Beam on time was longest for CyberKnife (30.25 + 7.32 minutes), followed by intensity-modulated radiation therapy (2.95 + 0.85 minutes), non-coplanar volumetric modulated arc therapy (2.61 + 0.07 minutes), and coplanar volumetric modulated arc therapy (2.30 + 0.23 minutes). Conclusion: For brain metastases far away from organs-at-risk, noncoplanar volumetric modulated arc therapy generated more rapid dose falloff and higher conformity compared to intensity-modulated radiation therapy and coplanar volumetric modulated arc therapy. Noncoplanar volumetric modulated arc therapy provided a comparable dose falloff with CyberKnife at high-dose level and a slower dose falloff than CyberKnife at moderate-to-low dose level. Noncoplanar volumetric modulated arc therapy plans had less monitor units and shorter beam on time than CyberKnife plans.