Clinical outcomes and relevance of composite V12 Gy in patients with four or more brain metastases treated with single fraction stereotactic radiosurgery

Objective: Tissue V12Gy (total brain volume receiving 12Gy including target) can predict for late toxicity in single target benign disease treated with stereotactic radiosurgery(SRS). The value of this metric remains uncertain for multiple brain metastases. This retrospective cohort study reports the outcomes and to evaluate predictors of toxicity in patients with four or more brain metastases treated with single fraction SRS.

Methods: 226 patients with 2160 metastases treated from 2014-21 were retrospectively studied. Symptomatic late toxicity (new/progressive neurological symptoms ≥ 3 months post SRS) with MRI changes suggestive of treatment-effect were analysed. Kaplan-Meier and competing risk analysis was used to assess survival and toxicity respectively.

Results: Median number of metastases/patient was 6 (range 4-41) and median composite tissue V12Gy (inclusive of PTV) was 11.3cc (IQR 6.1cc - 17.1cc). 16/226 patients developed symptomatic late R-AE and cumulative incidence was 4.9% at 1 year and 6.9% at 2 years. Total target volume was significantly predictive of the risk of late R-AE. Volume of the largest lesion, V12Gy and V15Gy did not predict for late R-AE, but plotted graphs showed suggestions of linear relationships between dosimetric parameters and late R-AE.

Conclusion: Within the limitations of this study, the cumulative incidence of symptomatic toxicity remains acceptable despite routinely accepting a composite tissue V12Gy in excess of 10cc to treat multiple brain metastases.

Advances in Knowledge: V12Gy has limitations as a plan-quality metric in multiple brain metastases treated with SRS. There is insufficient evidence to have a defined target limit as <10cc.