Malignant Melanoma Presenting as Multiple Lytic Bone Lesions Masquerading as Carcinoma of Unknown Primary on Clinical and Histopathological Examination

Abstract

Malignant melanoma presenting as multiple lytic bone lesions masquerading as carcinoma of unknown primary on clinical and histopathological examination. Malignant melanomas are aggressive tumours with frequent metastasis to lymph nodes and viscera. However, patients presenting with osseous metastasis of melanoma with no known primary source is relatively rare. The terminology “metastases of unknown origin” is used when the site of the primary neoplasm cannot be identified at the time of diagnosis despite a thorough history, physical examination, appropriate laboratory testing and imaging studies. Metastatic melanoma can have varied presentations. Herein, this is a case report of a 63-year-old female who presented with swelling in the occipital region of eight-month duration. Examination showed bony hard fixed swellings in the occipital and mastoid regions. Computerised Tomography scan (CT scan) showed lytic bone destruction in the occipital and mastoid part of right temporal bone, associated with soft tissue mass. With the differential diagnosis of metastatic carcinoma and myeloma, biopsy was taken which showed infiltrating neoplasm with plasmacytoid cells in cords and in vague nests surrounded by fibrosis. The neoplastic cells were Cytokeratin negative, Cluster of Differentiation (CD) 138 negative and showed patchy nuclear positivity for S100. On subsequent examination, the neoplastic cells showed diffuse strong positivity for HMB 45 (Human Melanoma Black) and patchy moderate positivity for Melan A confirming the diagnosis of malignant melanoma. Primary lesion could not be detected even after a detailed past medical and surgical history, physical examination and radiological investigations. The main causes of skull lytic lesions in adults are metastatic carcinoma and plasma cell myeloma. Possibility of metastatic melanoma should also be considered in cases of skull bone lesions as melanomas could exhibit varying cellular morphology mimicking carcinoma, sarcoma or haematolymphoid neoplasms.