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Metaplastic carcinoma (MC) is a highly malignant tumor that is mainly classified as a triple-negative subtype exhibiting basal markers on immunohistochemical testing. Immunohistochemical findings of S-100 and SOX10 are useful and valuable in the differential diagnosis of Schwannian, melanocytic, and myoepithelial/basal cell epithelial neoplasms, particularly of unknown differentiation or origin. We report an extremely rare case of MC with a basal-HER2 phenotype (HER2 phenotype breast carcinoma with basal marker expression) that exhibited unusual S-100 and SOX10 immunoreactivity. A 54-year-old Japanese woman presented with a 2-month history of a painful, growing mass in her left breast. Core needle biopsy was performed and immunohistochemical results suggested poorly differentiated carcinoma or melanocytic neoplasm. Because the tumor was located in the breast, the immunohistochemical panel included the St. Gallen consensus markers, which showed focal expression of HER2 (score: 2+, equivocal) and a Ki-67 labeling index of 87.0%, but no expression of estrogen receptors or progesterone receptors. Fluorescence in situ hybridization identified HER2 gene amplification based on the 2013 ASCO–CAP HER2 Test Guidelines (HER2/CEP17 ratio: 3:3; HER2 copy number: 14.7 per cell). The histopathological results suggested that this was a primary breast carcinoma of a “basal-HER2” phenotype. After histological examination of the resected specimen, the tumor was ultimately diagnosed as metaplastic carcinoma. Tumors like this could potentially be mis-diagnosed as other S-100-positive and SOX10-positive malignant neoplasms based on limited sampling, such as that occurring with core needle biopsies. This case highlighted the importance of performing careful histological examinations of HER2 status to correctly diagnose a primary breast neoplasm.

Objectives: To report three cases of primary squamous cell carcinoma of the breast with an unusual "basal-HER2" phenotype., Methods: Clinical data were analyzed. Morphological features were observed. Immunohistochemical study for ER, PR, HER2, Ki-67, CK 5/6, CK10/13, CK14, EGFR, P63 and FISH detection of HER2 gene amplification were performed., Results: Three patients were all female with 26, 57 and 66 years old. The tumors were 3 cm, 4 cm and 5 cm in size respectively. Morphologically, all three tumors were pure squamous cell carcinoma and entirely composed metaplastic squamous cells. Two tumors were moderately differentiated and one was poorly differentiated. All three patients were positive for P63 or CK10/13. All three tumors exhibited basal-HER2 phenotype: negative for ER and PR, positive for HER2 protein and HER2 gene amplification, and positive for at least two basal markers., Conclusions: SCC with basal-HER2 phenotype is an extremely rare subset of breast carcinoma. Since it may have worse prognosis than typical basal-like SCC, recognization of this unusual SCC in routine work may have obvious clinical significance.