Introduction/Background*Breast cancer is one of the most frequent tumors in the female population and can be associated with the detection of pathogenic germ-line BRCA1/2 mutation, which leads to close monitoring of patients and the consideration of prophylactic surgery. Occult synchronous breast cancer occurs in approximately 1-3% of patients and is detected incidentally.

The aim of our study is to determine the incidence of occult breast carcinoma in patients with BRCA ½ carriers who accept prophylactic mastectomy as risk reduction surgery.

Methodology Retrospective analysis of all patients assessed in the Breast Pathology Unit of the 12 de Octubre Hospital between January 2011 and January 2021, with a diagnosis of germ-line BRCA ½ mutation and who underwent breast cancer risk-reducing surgery. The occurrence of occult carcinoma was analysed. A descriptive study of these patients was performed. All statistical analysis was performed with Stata/IC 13.0 for Windows.

Result(s)*During the study period a total of 168 patients with BRCA ½ mutation were diagnosed and breast risk-reducing surgery was performed in 81 of them (48.2%).

In 61.7% (n=50) of the cases, prophylactic surgery was performed after the diagnosis of bilateral breast cancer. Bilateral breast and in 7.4 (n=6) ovarian cancer. In 58.0% (n=47) the mutation was BRCA 1 and in 42% (n=34) BRCA 2. In 39.5% (n=32) the mastectomy performed was nipple sparing and in 60.5% (n=49) simple. Breast reconstruction was performed after surgery in 93.8% (n=76) of cases. Definitive histopathological examination showed ductal carcinoma insitu in 3.7% (n=3) of cases, and infiltrating carcinoma in 1.2% (n=1).

Conclusion*In our case series, approximately half of the patients carrying BRCA mutation have opted for breast cancer risk-reducing surgery, with a proportion of incidental carcinomas between 1-3% in the mastectomy surgical specimens. Therefore, we can conclude that in patients carrying BRCA1/2 mutation, prophylactic mastectomy is effective in reducing the risk of breast cancer.