Nodal Merkel Cell Carcinoma in Head and Neck Lesions with an Unknown Primary: A Case Report in Light of the Literature

Merkel cell carcinoma (MCC) is a rare but aggressive neuroendocrine skin cancer. To diagnose nodal MCC with an unknown primary disease is challenging, and it has to be separated from other nodal metastatic neoplasms. We report a unique case of nodal MCC in head and neck lesions with an unknown primary. A 70-year-old woman was admitted to our department with a right submandibular mass. Fine needle aspiration biopsy was performed and indicated malignancy. F-18-fluorodeoxyglucose positron emission tomography (PET) demonstrated abnormal accumulation in the right submandibular lymph node, right palatine tonsil, and right thyroid gland. For diagnostics and treatment, bilateral selective neck lymph node dissection, right tonsillectomy, and right thyroidectomy were performed. Histopathological examination revealed that most parts of the submandibular lymph node were occupied by diffuse sheets of tumor cells. Contrary to our expectation, malignant cells were not detected in the right palatine tonsil and right thyroid. Immunohistochemistry demonstrated a marked positive reaction for AE1/AE3, chromogranin A, synaptophysin, cytokeratin 20 (CK20) and CD56 and a negative reaction for vimentin, leucocyte common antigen (LCA), thyroid transcription factor-1 (TTF1) and cytokeratin 7 (CK7) in the tumor cells. Immunostaining of Merkel cell polyomavirus-large T antigen (MCPyV-LT) showed a positive reaction and MCPyV-positive MCCs were assessed by PCR analysis, demonstrating that viral copy number was 12.8 copies per cell. These histological findings confirmed the diagnosis of Merkel cell carcinoma of the lymph node. In cases of tumors in the lymph node with a neuroendocrine appearance in head and neck lesions, it is necessary to eliminate the possibility of metastasis from MCC.