Apocrine Ductal Carcinoma in situ Ex Pleomorphic Adenoma of the Breast: A Rare Case Report

Pleomorphic adenoma (PA) of the breast is an extremely rare benign tumor, and malignant transformation within PA is exceptional. Only a few cases have been reported, and none have described apocrine ductal carcinoma in situ (DCIS) arising in PA. We report a case of apocrine-type DCIS considered to have developed from mammary PA.

A 75-year-old woman presented with a 2.0-cm palpable mass in the left breast with nipple inversion. Mammography showed a well-circumscribed 24-mm mass and an adjacent 11-mm lesion with irregular margins near the nipple. MRI demonstrated a cystic lesion with an enhancing mass on the nipple side and nipple retraction toward the lesion. Cytology of cyst fluid revealed no malignant cells and was consistent with a benign hemorrhagic cyst. However, core needle biopsy of the adjacent lesion identified apocrine-type DCIS. Systemic evaluation showed no metastases, and the clinical diagnosis was cTisN0M0, cStage 0 breast cancer. Because nipple involvement could not be excluded, nipple-sparing surgery was not recommended, and the patient underwent mastectomy. Histopathology revealed a well-defined, encapsulated PA containing an intraductal apocrine carcinoma component. The carcinoma cells showed abundant eosinophilic cytoplasm, high nuclear grade, and occasional mitoses without stromal invasion. Immunohistochemically, tumor cells were androgen receptor and Forkhead box protein A1-positive and estrogen- and progesterone receptor-negative, with a Ki-67 index of approximately 30%. The lesion was diagnosed as apocrine-type DCIS ex PA, with ductal extension toward the nipple corresponding to the clinical nipple retraction.

We describe the first reported case of apocrine-type DCIS arising in breast PA. Awareness of this rare entity is essential to avoid misdiagnosis and to clarify its clinicopathological characteristics.