Primary Nodal Merkel Cell Carcinoma of the Upper Extremity Masquerading as a Benign-Appearing Subcutaneous Nodule

Introduction Merkel cell carcinoma (MCC) is a rare, aggressive neuroendocrine carcinoma of the skin associated with high rates of recurrence, nodal involvement, and metastasis. While most cases arise in sun-exposed skin, a small subset presents solely within lymph nodes without an identifiable cutaneous primary, termed primary nodal MCC. These cases are exceedingly uncommon and can mimic benign soft tissue lesions, creating diagnostic challenges. Case Report A 56-year-old woman presented with a firm, mobile, non-tender 1 cm subcutaneous nodule on the right arm, initially presumed to be a lipoma. During excision, the dissection extended into a lymph node where tumor was identified. Histopathology demonstrated small blue cells with granular chromatin, scant cytoplasm, frequent mitoses, and focal necrosis. Immunohistochemistry showed paranuclear dot-like positivity for cytokeratin 20 (CK20), cytokeratin AE1/AE3 (AE1/AE3), neurofilament, and cluster of differentiation 99 (CD99), with diffuse synaptophysin positivity and negativity for cytokeratin 7 (CK7) and thyroid transcription factor-1 (TTF-1), confirming MCC. Deeper sections revealed tumor within a dense lymphoid background. Merkel cell polyomavirus testing was positive with elevated oncoprotein titers. Subsequent biopsy of the overlying skin showed only post-procedural changes, and comprehensive skin examination revealed no primary lesion. Discussion This case represents primary nodal MCC, a rare entity that may be mistaken for a benign subcutaneous mass. The absence of a cutaneous primary complicates diagnosis and staging. Recognition of this presentation and integration of histopathologic and immunohistochemical findings are essential for accurate diagnosis and appropriate management, particularly when lesions are incidentally identified during excision of presumed benign tumors.