Metaplastic carcinoma of the breast with melanocytic differentiation is an exceedingly rare tumor. We describe an additional metaplastic breast carcinoma with melanocytic differentiation that was initially diagnosed as malignant melanoma on core biopsy. The patient received neoadjuvant immunotherapy. The surgical specimen demonstrated closely associated epithelial and melanocytic components that had differing, but partially overlapping, immunohistochemical profiles. Molecular testing of the 2 components identified shared alterations in a few genes, supporting a clonal relationship. Following therapy, the melanocytic component showed an increase in tumor-infiltrating lymphocytes.
Ha et al. (2026) studied this question.