Background: There is limited information on the risk of metastatic breast cancer (MBC) to inform younger women, particularly those under 40 years.

Aims: We conducted a retrospective analysis of a population-based cohort study to describe the risk, site and prognosis of MBC in young women under 40 years with an initial diagnosis of non-metastatic BC and compare with older women.

Methods: Data was extracted from the NSW Central Cancer Registry and the Admitted Patient Data Collection database between 2001-2007. Main outcome measures were 5-year cumulative incidence of MBC, prognostic factors for MBC and overall survival (OS) from the date of MBC diagnosis.

Results: 395 (6%) of 6640 women with non-metastatic BC were <40 years. The 5-year cumulative incidence of MBC was 24% (95% CI 20%-29%) for women <40 years with non-metastatic BC, compared to 9% (95% CI 9-10%) for women ≥ 40years. Significant independent risk factors for MBC≤5 years were age <40, regional disease at diagnosis, low socioeconomic status and the presence of other non-breast primary. At first record of MBC, visceral sites were more common for women <40 years than ≥40 (54% vs. 43%; p=0.03). Median survival for women with MBC within 5 years was not significantly different between young and older women (<40 years 18months vs. ≥ 40 years 14months; log rank p=0.21).

Conclusions: Women with non-metastatic BC before age 40 have a higher 5-year risk of developing MBC than older women. There were no significant differences in median survival following MBC between young and older women.