Dedifferentiated Melanoma With Expression of Cytokeratin and GATA3 in a Patient With History of Breast Carcinoma.

Melanoma is one of the great mimickers in pathology because it has diverse morphologies and can be mistaken for carcinoma or sarcoma. In most cases, immunochemistry is helpful in supporting the diagnosis and excluding other differentials. However, metastatic melanoma may lose immunohistochemical melanocytic markers and express nonmelanocytic lineage markers, which often poses a diagnostic dilemma and may be misdiagnosed as a poorly differentiated carcinoma or sarcoma. We report the case of a 52-year-old woman who had a history of recurrent melanoma on her right shoulder with axillary lymph node metastasis (BRAF V600K-mutated melanoma) and right-side breast-invasive ductal carcinoma (stage pT1b N0sn). One year later, she presented with a left-sided chest wall mass and enlarging left axillary lymph nodes. Needle core biopsies were obtained from both lesions, and histologic examination showed a poorly differentiated tumor with pleomorphic/anaplastic morphology and necrosis. The tumor cells were strongly immunoreactive for GATA-3 without expression of melanocytic markers (S100, Melan A, HMB45, SOX10, MITF, and tyrosinase). The history of melanoma prompted molecular analysis, and the lesion was found to harbor the BRAF V600K mutation, consistent with metastatic dedifferentiated melanoma. Recognition of metastatic dedifferentiated melanoma is important to avoid misdiagnosis of carcinoma, especially in patients with a previous history of carcinoma.