Abstract
Undifferentiated melanoma (UM) is defined by the loss of classic morphologic and immunohistochemical melanocytic markers. Reports in the literature are rare and show that UM usually occurs as a metastasis in the setting of a known primary cutaneous melanoma. The most common mutations in UM include those involving BRAF, NRAS, and KIT, which are almost invariably present in the parent melanoma. Herein, we present a case of a primary sinonasal melanoma with metastatic UM resembling a primary bone tumor. A 72-year-old woman who presented with symptoms of chronic sinusitis was found to have a 5.5 cm destructive mass in the left nasal cavity. Biopsy showed sheets and nests of epithelioid cells with abundant clear cytoplasm, irregular nuclei, and prominent nucleoli. The tumor cells were positive for Melan-A, S100, and SOX-10, consistent with a diagnosis of mucosal melanoma. Microscopic examination of vertebral tissue following a pathologic T8 fracture demonstrated discohesive, sheet-like growth of neoplastic cells with marked pleomorphism and irregular nuclei, prominent nucleoli, and variable amounts of eosinophilic cytoplasm, admixed with hemorrhage, infiltrating histiocytes, and abundant osteoclast-like giant cells. There was focal staining for S100 in non-atypical cells and negative staining for SOX10, Melan-A, and tyrosinase. Retention of an unusual Q61H KRAS mutation that was also present in the primary lesion provided critical information for the diagnosis. Our report highlights the importance of considering mutational analysis to identify undifferentiated melanomas in patients with metastatic tumors which do not have the typical histopathologic and immunohistochemical features of melanoma.
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