Interinstitutional Plan Quality Assessment of 2 Linac-Based, Single-Isocenter, Multiple Metastasis Radiosurgery Techniques.

Purpose: Interest and application of stereotactic radiosurgery for multiple brain metastases continue to increase. Various planning systems are available for linear accelerator (linac)-based single-isocenter multiple metastasis radiosurgery. Two of the most advanced systems are BrainLAB Multiple Metastases Elements (MME), a dynamic conformal arc (DCA) approach, and Varian RapidArc (RA), a volumetric modulated arc therapy (VMAT) approach. In this work, we systematically compared plan quality between the 2 techniques.

Methods And Materials: Thirty patients with 4 to 10 metastases (217 total; median 7.5; V = 0.014 cm; V = 17.73 cm) were planned with both Varian RA and MME at 2 different institutions with extensive experience in each respective technique. All plans had a single isocenter and used Varian linac equipped with high-definition multileaf collimator. RA plans used 2 to 4 noncoplanar VMAT arcs with 10 MV flattening filter-free beam. MME plans used 4 to 9 noncoplanar DCAs and 6 MV flattening filter-free beam, (minimum planning target volume [PTV = 0.49 cm; PTV = 27.32 cm; PTV = 7.05 cm). Prescriptions were 14 to 24 Gy in a single fraction. Target coverage goal was 99% of volume receiving prescription dose (D99% ≥ 100%). Plans were evaluated by Radiation Therapy Oncology Group/Paddick conformity index (CI) score, 12 Gy volume (V), V, V, mean brain dose (D), and beam-on time.

Results: Conformity was favorable among RA plans (median: MME CI = 1.38; RA CI = 1.21; < .0001). V and V were lower for RA plans (median: MME V12 = 23.7 cm; RA V12 = 19.2 cm; = .0001; median: MME V = 53.6 cm; RA V = 44.1 cm;  = .024). V was lower for MME plans (median: MME V = 141.4 cm; RA V = 142.8 cm; = .009). Mean brain was lower for MME plans (median: MME D = 2.57 Gy; RA D = 2.76 Gy; < .0001).

Conclusions: For linac-based multiple metastasis stereotactic radiosurgery, RapidArc VMAT facilitates favorable conformity and V/V volume compared with the MME DCA plan. MME planning facilitates reduced dose spill at levels ≤V.