Malignant transformation of breast ductal adenoma: a diagnostic pitfall.

We present what is believed to be the first report of heterogeneous carcinoma arising from breast ductal adenoma. A 57-year-old woman presented with a nodule in her right breast. Histological examination of a vacuum-assisted biopsy specimen revealed epithelial tubular proliferation and papillary apocrine epithelium. The myoepithelial cells lining the tubules were confirmed by immunohistochemistry. The nodule had increased in size 18 months later and tumorectomy was performed. The surgical specimen revealed proliferating apocrine epithelium with sheet-like and cribriform architecture within a mammary duct. Some myoepithelial cells showed irregular proliferation around the tubular epithelium. All three components, including apocrine, myoepithelial and glandular cells, showed prominent nuclear atypia and significant mitotic activity. The patient was diagnosed with malignant transformation of ductal adenoma. The malignant potential of ductal adenoma has not previously been discussed, but this heterogeneous carcinoma could represent a serious pitfall in the diagnosis of ductal adenomas.