Track: Case Reports

Abstract:

Merkel cell carcinoma is a rare neuroendocrine tumor of the skin, marked by high recurrence, nodal spread, and metastasis. While most cases arise in sun-exposed skin, a small subset presents solely within lymph nodes without an identifiable skin primary, termed primary nodal Merkel cell carcinoma. These are uncommon and diagnostically challenging. A 56-year-old woman presented with a firm, non-tender 1 cm subcutaneous nodule on the right arm, initially presumed to be a lipoma. During surgical excision, the dissection extended into a lymph node where tumor was identified. Histopathology showed small blue cells with granular chromatin, scant cytoplasm, frequent mitoses, apoptotic bodies, and focal necrosis. Immunohistochemistry was positive for CK20, AE1/AE3, synaptophysin, neurofilament, and CD99 in a paranuclear dot pattern, and negative for CK7 and TTF-1, confirming Merkel cell carcinoma. Deeper sections demonstrated tumor within a dense lymphoid background. Merkel polyomavirus VP1 capsid antibody was positive, and oncoprotein titer was elevated. After excision, a targeted biopsy of overlying skin revealed only post-procedural changes, confirming absence of a cutaneous primary. Given the intranodal location and absence of epidermal involvement, the lesion was classified as a primary nodal Merkel cell carcinoma. The patient underwent complete excision, sentinel lymph node biopsy, and was referred to oncology. Primary lesions are often mistaken for benign masses, but nodal-only disease is rare, particularly when found incidentally during excision of a presumed soft tissue lesion. These presentations are challenging due to the lack of a visible primary site, and recognition is critical for accurate staging, prognosis, and management.

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Matthew Scholl