Survival advantage of locoregional and systemic therapy in oligometastatic breast cancer: an international retrospective cohort study (OLIGO-BC1)

Background: An international retrospective cohort study was conducted to clarify the survival advantage of combination therapy with locoregional and systemic therapy (ST) in oligometastatic breast cancer (BC). Methods: Patients with oligometastatic BC diagnosed from 2007 to 2012 were enrolled in center hospitals in China, Korea and Japan. It was defined as a low-volume metastatic disease at up to five sites and not necessarily in the same organ. Cases with brain, pleural, peritoneal and pericardial metastases were excluded. The primary endpoint was overall survival (OS) from the initial diagnosis of oligometastases. OS was summarized using the Kaplan–Meier method. A multivariable Cox regression model was used to estimate the hazard ratio (HR) for clinicopathological factors. Results: Among 1,295 cases registered from February 2018 to May 2019, 932 remained for analysis after the exclusion of unavailable cases and locoregional recurrence. One metastatic site was found in 400 cases, 2 in 243, 3 in 130, 4 in 86 and 5 in 73. At the median follow-up of 4.5 years, 5-year OS was 54.7% and 39.7% for 321 cases in the combination therapy group and 611 cases in the ST group, respectively. An adjusted HR was 0.66 (95% confidence interval: 0.55, 0.79). Some types of ST without chemotherapy alone, younger age, ECOG performance status 0, early-stage BC, non-triple negative subtype, fewer metastatic sites and longer duration of surgery to relapse were significantly favorable prognostic factors. Conclusion: Combination therapy may be considered for longer survival under some conditions in oligometastatic BC.