Merkel cell carcinoma of the inguinal lymph node with an unknown primary site

Merkel cell carcinoma (MCC) is an uncommon and aggressive primary neuroendocrine malignancy of the skin. Frequent local recurrences and disseminations to regional lymph nodes and distant organs are characteristic. MCC within the lymph nodes in the absence of a primary site is rare and has only been reported sporadically. We report a case of MCC presenting as a painless mass in the left inguinal area for 5 months in a 57-year-old man. The histopathology of the excised lesion revealed a poorly differentiated basophilic small cell tumor. The tumor cells were positive for cytokeratin 20 and CD56, negative for cytokeratin 7, thyroid transcription factor-1 and CDX-2. These immunohistochemical findings were consistent with the diagnosis of a metastatic MCC. Despite extensive clinical and radiological investigation, we failed to identify the origin of the tumor. Our case may represent a lymph node metastasis from an occult or regressed skin primary, but we cannot preclude the possibility of a primary nodal tumor.