Observational studies in metastatic breast cancer (MBC) show that long-term overall survival (OS) is associated with limited tumor burden, or oligo-MBC (OMBC). However, a uniform definition of OMBC is lacking. In this real-world nationwide cohort, we aimed to define the optimal OMBC threshold and factors associated with survival in patients with OMBC.
3,535 patients <80 years at diagnosis of de novo MBC in the Netherlands between January 2000 and December 2007 were included. Detailed clinical, therapy, and outcome data were collected from medical records of a sample of the patients. Using inverse-sampling-probability weighting (IPW) the analysis cohort (n = 3,447) was constructed. We assessed OS according to number of metastases at diagnosis to determine the optimal OMBC threshold. Next, we applied Cox-regression models with IPW to study associations with OS and progression-free survival (PFS) in OMBC. All statistical tests were two-sided.
Compared with >5 distant metastases, adjusted hazard ratios for OS (with 95% CI based on robust standard errors) for 1, 2–3, and 4–5 metastases were: 0.70 (0.52–0.96), 0.63 (0.45–0.89) and 0.91 (0.61–1.37), respectively. Ten-year OS-estimates for patients with ≤3 versus >3 metastases were 14.9% and 3.4% (P < .001). In multivariable analyses, pre-/perimenopausal status, absence of lung metastases and local therapy of metastases (surgery/radiotherapy) added to systemic therapy were statistically significantly associated with better OS and PFS in OMBC, independent of local therapy of the primary tumor.
OMBC defined as MBC limited to 1–3 metastases was associated with favorable OS. In OMBC local therapy of metastases was associated with better OS, particularly if patients were pre-/perimenopausal without lung metastases.