Trends in stage at diagnosis for young breast cancer patients in the United States.

PURPOSE: Previous studies reported increasing rates of metastatic breast cancer among young US women. However, these studies were based on limited geographic areas and did not account for the sharp decline in unknown-stage disease. In this study, we examined trends in early-onset breast cancer incidence rates by stage at diagnosis in a national dataset, after correcting for temporal changes in unstaged disease. METHODS: Using data from 42 states, covering 82% of the US population, we examined trends in incidence rates by stage at diagnosis and race/ethnicity in women ages 20-39 years. Stage was imputed for non-Hispanic (NH) white and NH black cases with missing information by distributing cases proportionally according to survival statistics. RESULTS: During 2001-2015, incidence rates of early-onset metastatic breast cancer increased sharply among NH white, NH black, Hispanic, and Asian/Pacific Islander (API) women. Increasing trends were also observed for local-stage disease (all racial/ethnic groups) and regional-stage disease (NH white and API). In contrast, rates decreased sharply for unstaged disease among all groups. After imputing stage for cases with missing information, the increasing trends for regional- and distant-stage disease in NH whites and local-stage disease in NH blacks were no longer statistically significant, but the increase in distant-stage disease in NH blacks was unchanged. CONCLUSIONS: After accounting for the sharp decline in unstaged cases, the increase in incidence rates for distant-stage disease became non-significant in NH whites but not in NH blacks. Future studies should consider accounting for temporal changes in unstaged disease when examining stage-specific incidence trends.