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

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.

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.

For GTV > 0.1 cm³, 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 cm³, 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.

The optimal IDL is 40%-50% for GTV > 0.1 cm³. 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.