Purpose/Objective(s): This study compares the dosimetry and efficiency of two stereotactic radiosurgery modalities for multiple brain metastases (single-isocenter RapidArc in and multi-isocenter Circular Cone Arc in iPlan).

Materials/Methods: Ten patients with three to five small brain metastases were used in this study. The size of targets varied from 0.13 to 12.91cc. SRS plans were made using both Circular Cone Arc in iPlan and a single-isocenter, four non-coplanar RapidArc in Eclipse.The prescription for all targets was standardized to 16Gy. Each plan was normalized for 100% prescription dose to 95% to 100% of target volume. Dosimetric parameters analyzed included Paddick conformity indices(CI), gradient indices(GI) and homogeneity indices(HI) for each target, dose falloff as well as the V₃,V₆,V₉,V₁₂ and V₁₆ isodose volumes.

Results: Compared with Cone Arc, four non-coplanar RapidArc improved plan CI (0.79±0.10 vs 0.50±0.22，p = 0.000) and HI(0.07±0.01 vs 0.15±0.07，p = 0.000), with no significant difference in 9Gy (65.60±33.36 vs 54.01±34.47, p = 0.059) and 12Gy isodose volume(32.00±16.87 vs 32.26±21.00, p = 0.445).Cone Arc plans achieved better low-dose spread(≤6Gy,p = 0.005).RapidArc plans were associated with markedly reduced monitor units（MUs).

Conclusion: For multiple brain metastases stereotactic radiosurgery, non-coplanar RapidArc(single-isocenter, four Arcs) has better CI and HI,V₁₂ are comparable. Because of its similar plan quality and increased delivery efficiency, single-isocenter RapidArc radiosurgery has more advantageous than multi-isocenter Circular Cone Arc radiosurgery for some patients.