Unusual variant of invasive ductal carcinoma suggesting metaplastic melanocytic differentiation

Background: Metaplastic carcinoma of the breast is a general definition referring to a heterogeneous group of neoplasms characterized by a mixture of adenocarcinoma with areas of spindled, squamous or mesenchymal differentiation. Metaplastic breast carcinoma comprising of both epithelial and melanocytic elements is rare, with just nine cases reported in medical literature so far. “metaplastic breast carcinoma with melanocytic differentiation” does not exist as a separate entity on the WHO classification. Case description: Patient reported is a woman diagnosed with having invasive ductal carcinoma in 1997, with new skin lesions appearing in 2009 and 2019. These lesions were biopsied and reviewed for histopathology. On microscopic examination, we observed skin involved by a malignant tumor, showing clusters of atypical cells with solid architecture and focal duct differentiation. Immunohistochemistry revealed tumor cells were positive for breast and melanocytic markers. Given the patient's clinical history, the possibility of metaplastic breast carcinoma with focal melanocytic differentiation was suggested. Discussion: In our case report, we were able to show dermal breast lesions from the same patient during a 10-year span, showing change and evolution of the lesion to its current form. Recurrent biopsies of similar malignant lesions showing evolution have been described scarcely in the medical literature. From immune-histochemical stains performed on blocks obtained from 2009, we were able to show the development of melanocytic markers over time in similar samples resected ten years apart.