Metastatic breast cancer (MBC) is a life-threatening disease, and although some data suggest a trend in survival improvement, it has not yet been unequivocally demonstrated. This study aimed to evaluate the overall survival (OS) of MBC patients, assessing its correlation with prognostic factors.

COSMO (Checking Overall Survival in a MBC Observational study) is an Italian longitudinal retrospective multicenter study that enrolled patients with MBC diagnosed between 2000 and 2008. The primary objective was to detect a temporal difference in OS; the secondary objective was to identify prognostic factors as causal factors of the temporal variation in OS.

A total of 3721 of 3930 patients from 31 centers were distributed in 3 periods: 886 (23.8%), 1302 (35.0%), and 1533 (41.2%) in 2000-2002, 2003-2005, and 2006-2008, respectively. With a median follow-up of 9.3 years, median OS was 2.8 years (95% confidence interval, 2.6-2.9). No difference in OS was found in the 3 cohorts (P for trend = .563). The worst prognosis was observed for patients with triple-negative MBC (OS, 1.5 years) and for those with central nervous system metastases (1.7 years); the best prognosis was observed in those with bone metastases or nonvisceral disease (3.4 and 3.2 years, respectively) and in patients with a disease-free interval, defined as the time between resection of the primary malignancy and diagnosis of MBC, of > 2 years (3 years).

The COSMO study found improvement in OS between 2000 and 2008. Molecular subtype remained the strongest prognostic factor, and the role of other prognostic factors was confirmed, in particular disease-free interval, site of metastasis, and age.