Linac-based radiosurgery for multiple brain metastases: Comparison between two mono-isocenter techniques with multiple non-coplanar arcs

Abstract

Background and purpose: Three mono-isocenter techniques with multiple non-coplanar arcs are nowadays clinically available for linac-based stereotactic radiosurgery (SRS) of multiple brain metastases (BM): HyperArc (HA), Multiple Brain Mets (MBM), and Monaco-HDRS. Two of them, HA and MBM, are here compared in terms of plan-quality, and dosimetric consistency between planning and delivering.

Materials and methods: For 20 patients with multiple BM (2-10), treated by mono-isocenter volumetric modulated arc therapy (VMAT) HA plans, mono-isocenter MBM dynamic conformal arc plans were generated. Prescription dose (Dp) was 18-25 Gy, for single-fraction, and 21-27 Gy, for three-fractions. Mean overall Planning Target Volume (PTV), expanded by 2 mm from each lesion, was 9.6 cm³ (0.5-27.9 cm³). Dose normalization of 100%Dp at 98%PTV was adopted. Plan-quality was compared by the Paddick conformity (CI) and gradient (GI) index, for the target, mean dose and V₁₂ volume, for the healthy brain, and number of monitor units (MU). Further, verification dosimetry by radiochromic films was performed for each plan, thus comparing also, by γ-analysis, the consistency between in-phantom computed and measured dose distributions.

Results: CI significantly improved for HA plans, changing on average from 0.75 (MBM) to 0.94 (HA) (p < .001). No significant differences between HA and MBM plans were computed for GI (p = .216), and for mean dose (p = .436) and V₁₂ (p = .062) to the healthy brain; although V₁₂ increased on average from 23.7 cm³ (HA) to 37.3 cm³ (MBM). No significant difference resulted for MU (p = .107), whereas γ (1 mm, 3%) and γ (2 mm, 2%) passing-rates significantly improved for HA plans (p = .006; p = .023).

Conclusions: HA plans assured a higher CI, while no significant difference resulted for any of the other planning metrics. Although on average slightly higher for HA plans, the dosimetric consistency between planned and delivered was satisfactory from both techniques. Hence, our judgement of near equal plan-quality from HA and MBM SRS-techniques.

Keywords: Brain metastases; HyperArc; MultipleBrainMets; SRS; VMAT.