What is the optimal isodose line for stereotactic radiotherapy for single brain metastases using HyperArc?

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.
(© 2024 The Author(s). Journal of Applied Clinical Medical Physics published by Wiley Periodicals, LLC on behalf of The American Association of Physicists in Medicine.)