Primary Acantholytic Squamous Cell Carcinoma of the Breast With Basal-Like Immunophenotype and Low Proliferation: A Case Report and Literature Review

Background Primary squamous cell carcinoma of the breast is an extremely rare subtype of metaplastic breast carcinoma, often showing aggressive clinical behavior and posing diagnostic challenges, particularly when acantholysis is present and mimics vascular neoplasms. Patient presentation We report a 49-year-old woman presenting with a rapidly growing palpable mass in the outer quadrant of the left breast. Initial cytology yielded keratin debris interpreted as benign. Following inflammatory changes, the lesion was excised with a clinical pre-diagnosis of granulomatous mastitis. Gross examination revealed an 8 × 7 × 7 cm partial mastectomy specimen containing a 6 × 5 × 4 cm firm, cystically and hemorrhagically degenerated, irregularly circumscribed tumor adjacent to but not continuous with the overlying skin. Microscopically, the tumor consisted of infiltrating nests of squamous epithelial cells with widespread acantholysis and single-cell keratinization, moderate nuclear atypia, and absence of lymphovascular or perineural invasion. Associated ductal carcinoma in situ was identified. Immunohistochemically, the tumor cells expressed pan-keratin, keratin 5/6, keratin 14, p63, CD10 (MME), and EGFR, while ER, PR, HER2, and GATA3 were negative. Ki-67 (MKI67) proliferation index was approximately 10%. Final diagnosis was primary acantholytic squamous cell carcinoma of the breast. Conclusion This patient highlights the importance of thorough histopathologic and immunohistochemical evaluation to distinguish acantholytic squamous cell carcinoma of the breast from vascular mimics and other metaplastic carcinomas. The basal-like, hormone-receptor negative, HER2-negative, and low proliferative profile in this tumor underscores the heterogeneity of primary squamous cell carcinoma of the breast and the need for individualized treatment strategies.