Linac-based VMAT radiosurgery for multiple brain lesions: comparison between a conventional multi-isocenter approach and a new dedicated mono-isocenter technique

Background: Linac-based stereotactic radiosurgery or fractionated stereotactic radiotherapy (SRS/FSRT) of multiple brain lesions using volumetric modulated arc therapy (VMAT) is typically performed by a multiple-isocenter approach, i.e. one isocenter per lesion, which is time-demanding for the need of independent setup verifications of each isocenter. Here, we present our initial experience with a new dedicated mono-isocenter technique with multiple non-coplanar arcs (HyperArc™, Varian Inc.) in terms of a plan comparison with a multiple-isocenter VMAT approach.

Methods: From August 2017 to October 2017, 20 patients with multiple brain metastases (mean 5, range 2-10) have been treated by HyperArc in 1-3 fractions. The prescribed doses (Dp) were 18-25 Gy in single-fraction, and 21-27 Gy in three-fractions. Planning Target Volume (PTV), defined by a 2 mm isotropic margin from each lesion, had mean dimension of 9.6 cm³ (range 0.5-27.9 cm³). Mono-isocenter HyperArc VMAT plans (HA) with 5 non-coplanar 180°-arcs (couch at 0°, ±45°, ±90°) were generated and compared to multiple-isocenter VMAT plans (RA) with 2 coplanar 360°-arcs per isocenter. A dose normalization of 100%Dp at 98%PTV was adopted, while D_2%(PTV) < 150%D_p was accepted. All plans had to respect the constraints on maximum dose to the brainstem (D_0.5cm3 < 18 Gy) as well as to the optical nerves/chiasm, eyes and lenses (D_0.5cm3 < 15 Gy). HA and RA plans were compared in terms of dose-volume metrics, by Paddick conformity (CI) and gradient (GI) index and by V₁₂ and mean dose to the brain-minus-PTV, and in terms of MU and overall treatment time (OTT) per fraction. OTT was measured for HA treatments, whereas for RA plans OTT was estimated by assuming 3 min. For initial patient setup plus 5 min. For each CBCT-guided setup correction per isocenter.

Results: Significant variations in favour of HA plans were computed for both target dose indexes, CI (p < .01) and GI (p < .01). The lower GI in HA plans was the likely cause of the significant reduction in V₁₂ to the brain-minus-PTV (p = .023). Although at low doses, below 2-5 Gy, the sparing of the brain-minus-PTV was in favour of RA plans, no significant difference in terms of mean doses to the brain-minus-PTV was observed between the two groups (p = .31). Finally, both MU (p < .01) and OTT (p < .01) were significantly reduced by HyperArc plans.

Conclusions: For linac-based SRS/FSRT of multiple brain lesions, HyperArc plans assured a higher CI and a lower GI than standard multiple-isocenter VMAT plans. This is consistent with the computed reduction in V₁₂ to the brain-minus-PTV. Finally, HyperArc treatments were completed within a typical 20 min. time slot, with a significant time reduction with respect to the expected duration of multiple-isocenters VMAT.