ABSTRACT
Objective: To investigate the histopathological and immunohistochemical characteristics of benign apocrine cystic papillary hyperplasia of the breast with loss of myoepithelial cell layer. Methods: The clinical data, histopathological features and immunohistochemical profile of patients with benign apocrine cystic papillary hyperplasia of breast with loss of myoepithelial cell layer from January 2016 to December 2021 were examined, in which six patients were identified. Results: All six patients were female, aged 36-61 years (median 46 years), who presented with a breast mass; three cases were from the left breast and three cases were from the right breast. Microscopic examination of all cases showed breast hyperplasia with apocrine cysts, accompanied by different degrees of micropapillary and papillary hyperplasia of apocrine cells. One case was associated with lobular carcinoma in situ, and one case was associated with apocrine ductal carcinoma in situ with intraductal dissemination in adenosis. Immunohistochemical staining of CK5/6, p63, SMA, SMMHC, Calponin and CD10 showed complete absence of myoepithelial cell layer surrounding ducts in apocrine cystic papillary hyperplasia. Conclusions: The myoepithelial cells of apocrine cystic papillary hyperplasia of the breast may undergo abnormal changes and may even be completely lost. The diagnosis should be comprehensively considered along with cytomorphological and histological features to avoid overdiagnosis.