Hybrid Planning for Complex Brain Metastases in Linear Accelerator (LINAC)-Based Stereotactic Radiosurgery: A...

Dynamic conformal arc (DCA) and volumetric modulated arc therapy (VMAT) are widely used for linear accelerator-based single-isocenter multi-target stereotactic radiosurgery (SRS) in treating multiple brain metastases. DCA offers advantages in delivery efficiency, quality assurance, and dose gradient steepness. However, DCA produces suboptimal conformity for challenging targets, including large postoperative cavities, irregularly shaped lesions, or tumors adjacent to critical organs-at-risk (OARs). VMAT improves conformity for complex targets but may be unnecessary for simple lesions within the same treatment plan. BrainLAB, Munich, Germany, recently developed a hybrid planning technique that applies VMAT arcs to one challenging lesion while using DCA for the remaining targets within a single plan.

Ten patients with 73 planning target volumes (PTVs) were retrospectively replanned using four techniques: DCA, Hybrid (BrainLAB Elements, v4.0), Varian RapidArc® (RA), and HyperArc (HA). Dosimetric endpoints included the Radiation Therapy Oncology Group (RTOG) conformity index (CI), Paddick conformity index (PCI), gradient index (GI), normal brain volume exposures (V23-V5 Gy), and maximum doses to critical organs at risk (OARs). Efficiency metrics included monitor units and beam-on time.

All techniques achieved ≥95% PTV coverage. VMAT-based plans (RA, HA) demonstrated superior conformity (median CI 1.03-1.11, PCI 0.85-0.90) compared with DCA (CI 1.41, PCI 0.68). Hybrid planning (CI 1.40, PCI 0.71) improved overall conformity versus DCA and achieved VMAT-like performance for VMAT-applied targets (CI 1.07, PCI 0.89). DCA exhibited the lowest GI; Hybrid maintained the low GI for VMAT-applied targets, while improving conformity. HA minimized normal brain exposure, while RA produced highest V5-V8 Gy. Hybrid resembled DCA in high-dose volumes while avoiding RA's extensive low-dose exposure. OAR maximum doses showed no significant differences. Hybrid had the longest beam-on time due to additional arcs.

Hybrid planning enhances conformity for challenging targets while preserving favorable DCA characteristics for remaining lesions. Despite longer beam-on times, Hybrid offers a practical solution for single-isocenter multi-target SRS cases involving irregular cavities or OAR-adjacent lesions.