Conditional Survival of Patients With Extracranial Oligometastatic Treated With Stereotactic Body Radiation Therapy: An International Consortium Study

Abstract

Prolonged survivorship is increasingly observed in patients with oligometastases (OM) treated with stereotactic body radiation therapy (SBRT). The purpose of this study was to study the conditional survival of patients with OM, which can provide more detailed prognostic information over time by considering time already survived. A multi-institutional database consisting of 1033 patients with OM (≤5 metastases) treated with SBRT between 2006 and 2017 was analyzed. Conditional overall survival (OS) and progression-free survival (PFS) in 3 years were obtained at multiple time points for all patients and by primary cancer type. Cox regression was used to determine trends in conditional OS and PFS. Changes in the predictors of OS and PFS over time were also determined by multivariable Cox regression. The median follow-up was 24 months (0.3-105 months). Three-year OS and PFS at baseline were 56.7% and 23.2%, respectively. The OS in 3 years conditional on having survived for 3, 6, 12, and 24 months did not significantly change over time (56.7%, 55.4%, 55.8%, and 50.6%, respectively; P=.60). In contrast, the probability of PFS in 3 years conditional on having survived progression-free for 3, 6, 12, and 24 months significantly increased over time (23.6%, 27.3%, 35.1%, and 48.8%, respectively; P < .001). When stratified by primary site, conditional PFS significantly increased over time for patients with colorectal, breast, or kidney cancer. Conditional OS remained stable for patients with non-small cell lung cancer or kidney cancer but significantly decreased over time for patients with prostate, breast, or colorectal cancer. Changes in significant prognostic factors of OS and PFS over time were also observed. Analysis of conditional survival among patients with OM showed that as patients survived longer, their prognosis for further survival remained stable or decreased. However, patients who survived longer without disease progression had increased probability of PFS over time.