Bilateral Mastectomy and Breast Cancer Mortality.

Importance: The benefit of bilateral mastectomy for women with unilateral breast cancer in terms of deaths from breast cancer has not been shown.

Objectives: To estimate the 20-year cumulative risk of breast cancer mortality among women with stage 0 to stage III unilateral breast cancer according to the type of initial surgery performed.

Design, settings, and participants: This cohort study used the Surveillance, Epidemiology, and End Results (SEER) Program registry database to identify women with unilateral breast cancer (invasive and ductal carcinoma in situ) who were diagnosed from 2000 to 2019. Three closely matched cohorts of equal size were generated using 1:1:1 matching according to surgical approach. The cohorts were followed up for 20 years for contralateral breast cancer and for breast cancer mortality. The analysis compared the 20-year cumulative risk of breast cancer mortality for women treated with lumpectomy vs unilateral mastectomy vs bilateral mastectomy. Data were analyzed from October 2023 to February 2024.

Exposures: Type of breast surgery performed (lumpectomy, unilateral mastectomy, or bilateral mastectomy).

Main outcomes and measures: Contralateral breast cancer or breast cancer mortality during the 20-year follow-up period among the groups treated with lumpectomy vs unilateral mastectomy vs bilateral mastectomy.

Results: The study sample included 661 270 women with unilateral breast cancer (mean [SD] age, 58.7 [11.3] years). After matching, there were 36 028 women in each of the 3 treatment groups. During the 20-year follow-up, there were 766 contralateral breast cancers observed in the lumpectomy group, 728 contralateral breast cancers in the unilateral mastectomy group, and 97 contralateral cancers in the bilateral mastectomy group. The 20-year risk of contralateral breast cancer was 6.9% (95% CI, 6.1%-7.9%) in the lumpectomy-unilateral mastectomy group. The cumulative breast cancer mortality was 32.1% at 15 years after developing a contralateral cancer and was 14.5% for those who did not develop a contralateral cancer (hazard ratio, 4.00; 95% CI, 3.52-4.54, using contralateral breast cancer as a time-dependent covariate). Deaths from breast cancer totaled 3077 women (8.54%) in the lumpectomy group, 3269 women (9.07%) in the unilateral mastectomy group, and 3062 women (8.50%) in the bilateral mastectomy group.

Conclusions and relevance: This cohort study indicates that the risk of dying of breast cancer increases substantially after experiencing a contralateral breast cancer. Women with breast cancer treated with bilateral mastectomy had a greatly diminished risk of contralateral breast cancer; however, they experienced similar mortality rates as patients treated with lumpectomy or unilateral mastectomy.