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Case Reports

Abstract

Merkel cell carcinoma (MCC) is a rare, aggressive cutaneous neuroendocrine carcinoma, typically characterized by cytokeratin 20 (CK20) positivity in a perinuclear dot pattern. We present the case of a 65-year-old male with a two-month history of a progressively enlarging nodule on the right forehead. Histopathologic examination revealed a dermal-based basophilic tumor with granular chromatin. Immunohistochemical staining demonstrated CK7, pan-cytokeratin, and neurofilament staining in a paranuclear dot pattern. Staining for synaptophysin was positive. Notably, staining for CK20 was entirely negative, as was TTF-1. A diagnosis of CK-20 negative MCC was made. The lesion was excised via Mohs micrographic surgery with a 1 cm margin, followed by negative sentinel lymph node biopsy and adjuvant radiation therapy. CK20-negative MCC represents an uncommon diagnostic challenge, as this staining pattern is exceedingly rare and can mimic metastatic small cell lung carcinoma (SCLC). In this case, TTF-1 negativity and neurofilament positivity have utility in excluding SCLC.  Furthermore, CK20 negativity has been associated with a lower prevalence of Merkel cell polyomavirus, potentially indicating distinct tumorigenic pathways and prognostic implications. Awareness of such atypical staining patterns is critical for accurate diagnosis and appropriate clinical management. This case adds to the limited literature on CK20-negative, CK7-positive MCC and underscores the importance of broad immunohistochemical panels in evaluating cutaneous small round blue cell tumors.