Distant Metastasis Velocity as a Novel Prognostic Score for Overall Survival After Disease Progression Following Stereotactic Body Radiation Therapy for Oligometastatic Disease

Purpose: In patients with extracranial oligometastatic disease, distant failure (DF) after local ablative therapies is common. Prognostic scores to guide salvage treatment decision making are currently lacking. Analogous to brain metastasis velocity, we propose distant metastasis velocity (DMV) as a prognostic score for overall survival (OS) and widespread failure-free survival (WFFS) after DF following metastasis-directed stereotactic body radiation therapy (SBRT).

Methods and materials: Patients with ≤5 metastases from solid organ malignancies treated with SBRT to all lesions at our institution from 2014 to 2019 were screened, and patients who developed DF were included in this retrospective analysis. DMV was defined as metastases per month, determined at DF, and transformed into a 3-level categorical variable with cut points that minimized the log-rank P value for OS. Simple and multiple linear regression was used to predict DMV based on different patient and treatment variables. The association of DMV and other variables with OS was studied by univariable and multivariable Cox regression.

Results: Three hundred eighty-five patients were screened, of which 303 developed DF and were included. The median DMV was 0.7 metastases per month. Patients with <0.5, 0.5 to 1.5, and >1.5 metastases per month were classified as low, intermediate, and high DMV, and had a median OS of 37.1, 26.7, and 16.8 months, respectively (P < .0001). On multivariable analysis, DMV was a strong independent predictor of OS, with a hazard ratio of 0.31 for low (P < .001) compared with high DMV. Lower DMV was significantly associated with longer WFFS (P = .04). The cumulative metastases volume at baseline (regression coefficient β = 0.03, P = .04) and oligoprogressive/-persistent disease (β = 1.91, P = .10) predicted higher DMV.

Conclusions: DMV is a novel metric strongly associated with OS and WFFS after DF following SBRT in patients with oligometastatic disease and should be evaluated for decision making about the optimal multimodality salvage treatment strategy. The prognostic value of DMV should be validated in prospective studies.