[Low-grade adenosquamous carcinoma of the breast arising from benign sclerosing lesions: a clinicopathological analysis].

<b>Objective:</b> To investigate the clinicopathological features, immunohistochemical characteristics, and differential diagnosis of low-grade adenosquamous carcinoma of the breast arising from benign sclerosing lesions. <b>Methods:</b> Twelve cases of low-grade adenosquamous carcinoma arising from benign sclerosing lesions of the breast were collected, which were diagnosed from January 2010 to December 2023 at the Seventh Medical Center of the Chinese People's Liberation Army General Hospital. Their clinical manifestations, histopathological morphology, and immunohistochemical characteristics were analyzed and related literatures were reviewed. <b>Results:</b> All the 12 patients were females with a median age of 42 years (21-60 years). Five of the 12 cases had coexisting complex sclerosing lesions, 5 with sclerosing adenosis, 1 with sclerosing intraductal papilloma, and 1 with ductal adenoma. Microscopically, low-grade adenosquamous carcinoma grew infiltratively in multinodular and sclerosing lesions. The carcinomatous component was characterized by small irregular glandular structures, tubular formations, solid nests, clusters, or a single tumor cell. The epithelium showed varying degrees of squamous differentiation. The carcinoma was surrounded by fibroadenomatoid and desmoplastic lesions. The invasive neoplastic component typically infiltrated normal breast structures, and might infiltrate nerves and adipose tissue. There were lymphocytic aggregates commonly seen at the periphery. By immunohistochemistry, the tumor cells of all 12 cases showed diffuse and strong immunopositivity for CK5/6; negative expression of ER, PR and HER2; and variable expression of myoepithelial markers such as SMA, calponin, SMMHC and others. There was varied staining pattern of tumor cells for p63. CK8/18 (or CK7) was variably positive or negative. The proliferative index measured by Ki-67 was low. <b>Conclusions:</b> Low-grade adenosquamous carcinoma of the breast is a rare variant, which is found to coexist with other benign sclerosing lesions and can be easily missed and/or misdiagnosed. Their invasive growth pattern, presence of sweat duct-like structures and immunophenotypic profile are key features for appropriate diagnosis.