Early HER2-positive breast cancer arising from a fibroadenoma: a case report.

Breast cancer arising from fibroadenoma (FA) is rare, in which almost all reported cases are human epidermal growth factor receptor 2 (HER2)-negative. This is the first report to describe a case of HER2-positive breast cancer arising from FA that was treated with chemotherapy plus anti-HER2 therapy. In this early case, upfront surgery outcomes guided the selection of appropriate systemic therapy. A 31-year-old woman previously diagnosed with FA experienced tumor growth. Core needle biopsy and imaging studies confirmed a diagnosis of stage IIA HER2-positive invasive ductal carcinoma (IDC) with no evidence of lymph node metastasis (cT2N0M0). Breast-conserving surgery was performed. Pathological diagnosis revealed stage IA IDC with a predominant intraductal component (pT1aN0M0), arising from FA. In conclusion, we encountered an extremely rare case of HER2-positive breast cancer arising from FA in which pathological infiltration was difficult to predict based on preoperative imaging.