Spectrum of histopathologic findings in risk-reducing bilateral prophylactic mastectomy in patients with and without BRCA mutations.

Many germline mutations have been implicated in breast cancer pathogenesis and despite several studies on occult atypical lesions in prophylactic mastectomy specimens from patients with BRCA1/2 mutations, there are very limited data on other genes associated with increased breast cancer risk and the distribution of lesions in patients with hereditary breast cancer. We identified 207 patients who underwent bilateral prophylactic mastectomy due to germline mutations in BRCA1/2, PALB2, CHEK2, ATM, CDH1, PTEN, BARD1, or strong family history between 2015 and 2023. Patients with biopsy-proven past or current invasive breast carcinoma or carcinoma in-situ preoperatively were excluded. In addition to multiple benign lesions, the following atypical lesions were identified: flat epithelial atypia (16.9%), atypical ductal hyperplasia (14.0%), lobular neoplasia (14.0%), ductal carcinoma in-situ (4.3%), invasive ductal carcinoma (0.4%). Both low-grade and high-grade pathway lesions were identified in this cohort, and in a subset of patients, they co-occurred. The frequency of atypical lesions identified in patients with strong family history were comparable to those with proven germline mutation. PTEN immunohistochemistry showed loss of expression in ductal carcinoma in-situ and tubular adenomas in PTEN-mutant patients. Overall, findings from this cohort support the benefit of prophylactic mastectomy in patients with germline mutations and/or strong family history. Additionally, this is the first demonstration that PTEN immunohistochemistry may be helpful in identifying germline mutations in patients with atypical or neoplastic proliferations.