Mammary Carcinoma Arising in Microglandular Adenosis: A Report of Five Cases.

Mammary carcinoma arising in microglandular adenosis (MGA) is extremely rare, and MGA is regarded as a non-obligate precursor of triple-negative breast cancer. We report five cases of carcinoma arising in MGA of the breast. All cases showed a spectrum of proliferative lesions ranging from MGA to atypical MGA, ductal carcinoma in situ or invasive carcinoma. Immunohistochemically, all cases were triple-negative and expression of S-100 protein gradually decreased as the lesions progressed from MGA to atypical MGA and carcinoma. Three cases showed acinic cell differentiation with reactivity to α1-antitrypsin, and one case was metaplastic carcinoma. During clinical follow-up, one patient developed local recurrence. Carcinoma arising in MGA is a rare but distinct subset of triple-negative breast cancer with characteristic histologic and immunohistochemical findings.