Approximately 40,000 individuals die from metastatic breast cancer each year. We examined what fractions of annual breast cancer-specific death (BCSD) are due to stage I, II, III, IV disease and if these proportions changed over time.

We used data from SEER covering 1975 to 2017. After filtering for female sex at birth, one primary tumor type, surgery, AJCC (6th edition) stage > 0, no bilateral cancer, and survival data available, the final analysis included 972,763 patients. Temporal trends were assessed using a linear model and ANOVA test.

The contribution of stage I and II cancers to BCSD increased significantly from 16.2% to 23.1%, and from 30.7% to 39.5%, respectively between 2000 and 2017. The contribution of stages III and IV cancers decreased from 36.4% to 30.3%, and from 16.7% to 7.1%. In 2000, 0.92%, 4.0% and 10.7% of BCSD were due to T1a, T1b, and T1c node-negative cancers which increased significantly to 1.9%, 5.8%, and 14.7% by 2017. These temporal trends were similar for hormone receptor-positive and -negative cancers. The contribution of BCSD to all-cause mortality declined from 23.9% to 16.6% for stage I, and from 47.7% to 36.9% for stage II cancers by 2017.

Patients with stage I/II breast cancers have excellent prognosis, yet these cancers account for over 60% of current BCSD because of their large absolute numbers. To further reduce breast cancer death strategies are needed to identify and treat patients with stage I/II disease who remain at risk for recurrence.