OBJECTIVETo investigate the clinicopathologic and immunohistochemical features as well as the differential diagnoses of the solid variant of mammary adenoid cystic carcinoma with basaloid features.

METHODSClinical and pathological data were collected in four cases of the solid variant of mammary adenoid cystic carcinoma with basaloid features, and microscopic pathological examination and immunohistochemistry EnVision method were performed. The relevant literature was also reviewed.

RESULTSThe four patients were female, with age ranged from 46 - 65 years old (average 56 years) and the maximum tumor diameter ranged from 1.5 to 2.5 cm. Microscopically, the tumors exhibited a predominantly solid architecture with a myxoid or hyalinized stroma. The tumor cells showed moderate to marked nuclear atypia, and a basaloid appearance with scanty cytoplasm and inconspicuous nucleoli, and ≥ 5 mitotic figures per 10 high power fields. Glandular space embedded within tumor islands could be noticed. These spaces were genuine glandular structures and the cells lining these true glandular lumens had more abundant and eosinophilic cytoplasm. Pseudoglandular spaces of cribriform pattern or variable shape were also occasionally seen, and these cysts contained homogenous eosinophilic material. Focal necrosis was found. All cases were negative for ER, PR and HER2. Immunohistochemical staining for CK5/6, CK7 and CK14 was positive in the genuine glandular structures. All cases were positive for CD10, but also positive with varying intensity from weak to strong for vimentin and CD117. Staining for Ki-67 in three patients showed 10% - 50% positive.

CONCLUSIONSThe solid variant of mammary adenoid cystic carcinoma with basaloid features is a histologically distinctive and also a rare subset of the mammary adenoid cystic carcinoma. Awareness of its pathological features can help with the diagnosis as well as differential diagnosis. More cases are still needed for accurately assessing the prognosis of this particular tumor.