Abstract

Primary melanocytic tumors of the central nervous system (CNS) are exceptionally rare and can present significant diagnostic challenges when present outside the CNS.  We report a case of a 43-year-old female who presented to the ED with acute abdominal pain and was found to have an adnexal mass, which was resected. Gross examination revealed a cystic neoplasm containing a well-circumscribed, darkly pigmented multinodular lesion. Microscopic examination showed a mature cystic teratoma with a multinodular proliferation of epithelioid and spindled melanocytes completely surrounded by neural elements.  The melanocytes displayed mild to moderate nuclear enlargement, patchy moderate nuclear pleomorphism, increased pigmented cytoplasm, and rare mitotic figures.  The melanocytes did not infiltrate the surrounding neural tissue or the ovarian capsule.  By immunohistochemistry, the melanocytes expressed SOX10, Melan-A, and HMB45 and were PRAME-negative. The differential diagnosis included CNS melanocytoma, primary malignant melanoma arising in the mature cystic teratoma, metastatic melanoma to the ovary, and melanotic schwannoma. The patient had no known history of cutaneous melanoma. Molecular studies identified a GNAQ mutation. No BRAF, NRAS, KIT, TERT, or PRKAR1A mutations were detected. The GNAQ mutation and presence of adjacent leptomeningeal tissue supported the diagnosis of a circumscribed primary leptomeningeal melanocytic neoplasm arising in a mature cystic teratoma. To the best of our knowledge, this is the first reported case of its kind. This case illustrates an unusual presentation of a CNS melanocytoma and highlights the importance of the correlation between histological and molecular features in establishing the diagnosis.