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Abstract Background: Women with breast cancer and a BRCA1 mutation face a high risk of contralateral breast cancer. For this reason, many women with a unilateral breast cancer opt for bilateral mastectomies. Some will have a contralateral prophylactic mastectomy as a second surgery. However, it is not clear to what extent this operation impacts breast cancer mortality. The objective of the current study was to evaluate the differences in survival by surgical treatment in an international cohort of women with a BRCA1 mutation and unilateral breast cancer. Methods: Eligible participants were identified from a large international cohort of women with breast cancer and a BRCA1 mutation, from 26 collaborating centres. Patients with DCIS and stage IV breast cancer were excluded. Patients with synchronous bilateral cancer were excluded. Demographic data were patient reported and clinical and treatment data were collected from medical records. Women were followed from date of breast cancer diagnosis to either date of last follow-up or date of death. Results: There were 2482 eligible participants from 26 centres from 11 countries. The mean age of breast cancer diagnosis was 43.1 years (range 18-70 years). Among those who had a unilateral mastectomy or lumpectomy, the risk of contralateral breast cancer at 20 years was 27.5%. After experiencing a contralateral cancer the hazard ratio for breast cancer death was 2.14 (95%CI 1.43-3.18), P=0.0002. The fifteen-year breast cancer specific survival in the entire cohort was 82.9%. The survival was 78.7% for those who had a unilateral mastectomy, 86.2% for those who had a lumpectomy, and 88.7% for those who had bilateral mastectomies. 529 of the women who initially underwent a unilateral surgery subsequently had a contralateral or bilateral preventive mastectomy in the follow up period. 529 of the women with unilateral surgery had a contralateral or bilateral preventive mastectomy in the follow up period. After adjusting for age of diagnosis, tumor size, nodal status, chemotherapy (yes/no) and preventive mastectomy (time dependent) the hazard ratio for breast cancer mortality for bilateral surgery versus unilateral surgery was 0.78 (95% CI 0.55-1.13), p = 0.19. Discussion: Women with a BRCA1 mutation and breast cancer who develop a contralateral breast cancer have double the risk of mortality compared to women who do not develop contralateral breast cancer. We observed a small non-significant reduction in mortality for those who had bilateral mastectomies as initial treatment, but the cohort will require longer follow up for definitive results. Women with a BRCA1 mutation should be counselled on the risks of contralateral breast cancer and make surgical decisions with this knowledge. Citation Format: Kelly Metcalfe, Jan Lubinski, Jana Soukupová, Rinat Bernstein-Molho, Mina Lee, Evans Gareth, Arvids Irmejs, Robert Fruscio, Cynthia Villareal Garza, Joanne Ngeow, Jennifer Plichta, Rinat Yerushalmi, Steven Narod. Surgical Treatment of Women with Breast Cancer and a BRCA1 Mutation: An International Analysis of the Impact of Bilateral Mastectomy on Survival abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr GS02-04.
Metcalfe et al. (Thu,) studied this question.