Abstract
PURPOSE: The study aimed to investigate the optimal isodose line (IDL) in linear accelerator-based stereotactic radiotherapy for single brain metastasis, using HyperArc. We compared the dosimetric parameters for target and normal brain tissue among six plans with different IDLs. METHODS: This study included 30 patients with single brain metastasis. We retrospectively generated six plans for each tumor with different IDLs (80%, 70%, 60%, 50%, 40%, and 33%) using HyperArc. All treatment plans were normalized to the prescription dose of 35 Gy in five fractions which was covered by 95% of the planning target volume (PTV), defined by adding a 1.0 mm margin to the gross tumor volume (GTV). The dosimetric parameters were compared among the six plans. RESULTS: For GTV > 0.1 cm(3), the ratio of brain-GTV volumes receiving 25 Gy to PTV (V(25Gy)/PTV) was significantly lower at IDL 40%-70% than at IDL 80% and 33% (p < 0.01, retrospectively). For GTV < 0.1 cm(3), V(25Gy)/PTV decreased continuously as IDL decreased. The values of D(99%) and D(80%) for GTV increased with decreasing IDL. An IDL of 50% or less was required to achieve D(99%) of greater than 43 Gy and D(80%) of greater than 50 Gy. The mean values of D(99%) and D(80%) for IDL 50% were 44.3 and 51.9 Gy. CONCLUSION: The optimal IDL is 40%-50% for GTV > 0.1 cm(3). These lower IDLs could increase D(99%) and D(80%) of GTV while lowering V(25Gy) of normal brain tissue, which may help reduce the risk of radiation necrosis and improve local control.