LAUSR

Synchronous bilateral breast cancer is uncommon, and its pattern and incidence among younger women is unknown. Here we report the incidence, phenotypes, and long-term oncologic outcomes of bilateral breast cancer in women enrolled in the Young Women’s Breast Cancer Study (YWS). The YWS is a multi-center, prospective cohort study of women with breast cancer diagnosed at age ≤ 40 years. Those with synchronous bilateral breast cancer formed our study cohort. Tumor phenotypes were categorized as luminal A (hormone receptor (HR)+/HER2−/grade 1/2), luminal B (HR+ /HER2+ or HER2− and grade 3), HER2-enriched (HR−/HER2+), or basal-like (HR−/HER2−). Descriptive statistics were used to evaluate tumor phenotypes of bilateral cancers for concordance. Among 1302 patients enrolled in the YWS, 21 (1.6%) patients had synchronous bilateral disease. The median age of diagnosis was 38 years (range 18–40 years). Seventeen (81.0%) underwent genetic testing with 6 found to have pathogenic germline mutations in BRCA1, BRCA2, or TP53. The majority of patients (76.2%) underwent bilateral mastectomy. On pathology, 2 patients had bilateral in-situ disease, 6 had unilateral invasive and contralateral in-situ disease, and 13 had bilateral invasive disease. Of those with bilateral invasive disease, 10 (76.9%) had bilateral luminal tumors and, when fully characterized, 6 were of the same luminal subtype. Only 1 patient had bilateral basal-like breast cancer. At median follow-up of 8.2 years, 14 patients are alive with no recurrent disease. Bilateral breast cancer is uncommon among young women diagnosed with breast cancer at age ≤ 40. In our cohort, the majority of invasive tumors were of the luminal phenotype, though some differed by grade or HER2 status. These findings support the need for thorough pathologic workup of bilateral disease when it is found in young women with breast cancer to determine risk and tailor treatment.