Case Report: Axillary Presentation of Merkel Cell Carcinoma: A Rare Case Report

<ns3:p>Background Merkel Cell Carcinoma (MCC) is a rare, aggressive neuroendocrine skin malignancy. It is typically associated with sun-exposed areas in elderly, fair-skinned individuals. Pathogenesis involves either Merkel Cell Polyomavirus (MCPyV) integration or ultraviolet-induced genetic mutations. Due to rapid progression and high metastatic potential, early detection remains the cornerstone of clinical management. Morphological similarities to other “small-blue-round cell” malignancies require precise immunohistochemistry for confirmation. Case Presentation We report the case of a 58-year-old male who presented with a persistent, firm, painless mass in the left axillary region. While clinical assessment and PET/CT imaging identified hypermetabolic activity in the axillary region suspicious for lymph node metastasis, the lesion was identified as subcutaneous MCC rather than nodal involvement. A radical excision of the deep dermal mass and axillary lymph node dissection was performed. Histopathology revealed a malignant neoplasm with solid, trabecular, and nested growth patterns without lymph node parenchyma. The diagnosis of MCC was confirmed via an immunohistochemical panel showing diffuse positivity for neuroendocrine markers like Chromogranin A, Synaptophysin, and Neuron-Specific Enolase (NSE) along with a high Ki-67 proliferative index of 90%. Importantly, negative staining for Thyroid Transcription Factor 1 (TTF-1) and Cytokeratin 7 (CK7) and the absence of tumour formation on imaging studies allowed us to exclude metastatic pulmonary and adenocarcinomatous origins. Despite a localized recurrence requiring a secondary excision and adjuvant radiotherapy, the patient remains stable under active oncological surveillance. Conclusion This case illustrates the diagnostic complexity of MCC presenting as an occult primary in the axilla, where it often manifests as metastatic disease. It emphasizes that a lack of epidermal communication and an atypical anatomical site should not exclude MCC. Given the high recurrence risk, this report advocates for high clinical suspicion and a multidisciplinary management approach.</ns3:p>