A Rare Case of CK20/CK7 Double Negative Merkel Cell Carcinoma

The authors present a case of Merkel cell carcinoma (MCC) with unique immunohistochemical staining characteristics. A 57-year-old woman presented with a firm 0.3 cm tan papule on her left nasal-labial fold that was reportedly increasing in size and bleeding. She had a history of multiple head and neck actinic keratoses, papillary thyroid carcinoma, and a family history of an uncle with melanoma. The clinical differential diagnosis was "non-melanoma skin cancer." Histological examination showed a markedly atypical-appearing basaloid neoplasm, present mostly in the dermis, with focal pagetoid spread into the epidermis. The cells showed hyperchromatic-staining nuclei, crowding, nuclear molding, and scant cytoplasm with atypical mitoses. The findings were consistent with a malignant tumor, highly suspicious for MCC. A pancytokeratin stain was strongly positive and showed perinuclear dot-like positivity. CK20 and CK7 stains were both negative. Synaptophysin was strongly positive, chromogranin was focally positive, CD56 was weakly positive, and neurofilament was positive in a perinuclear dot-like pattern. TTF-1, PAX5, S100, and Melan-A were negative, arguing against metastatic small cell carcinoma of lung or thyroid, B-cell lymphoma, or melanoma, respectively. Although the CK20/CK7 double negativity is very unusual, the staining characteristics of this case are most consistent with a primary cutaneous MCC. Up to 10%-15% of MCCs can be CK20 negative, and those cases are typically CK7 positive. This case is unique, as a CK20/CK7 double negative case has not been previously reported; however, the diagnosis can still be rendered based on the clinical, histological, and other immunohistochemical findings.