title: A Rare Case of CK20/CK7 Double Negative Merkel Cell Carcinoma.
abstract: 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.
authors.affiliation: *Department of Pathology, University of California Los Angeles, Los Angeles, CA; †Southern California Permanente Medical Group, Department of Dermatology, Sunset Medical Center, Los Angeles, CA; and ‡Department of Pathology, Kaiser Southern California Permanente Medical Group, Los Angeles, CA.
authors.forename: Mary D
authors.initials: MD
authors.lastname: Le
authors.forename: Luke H
authors.initials: LH
authors.lastname: OʼSteen
authors.forename: David S
authors.initials: DS
authors.lastname: Cassarino
mesh_terms: D014408:Biomarkers, Tumor / Q000032:analysis; D002277:Carcinoma / Q000473:pathology; D015266:Carcinoma, Merkel Cell / Q000473:pathology*; D002291:Carcinoma, Papillary; D005260:Female; D006801:Humans; D007150:Immunohistochemistry; D053554:Keratin-20 / Q000032:analysis; D053552:Keratin-7 / Q000032:analysis; D055623:Keratosis, Actinic / Q000150:complications; D008875:Middle Aged; D016609:Neoplasms, Second Primary / Q000473:pathology; D012878:Skin Neoplasms / Q000473:pathology*; D000077273:Thyroid Cancer, Papillary; D013964:Thyroid Neoplasms / Q000473:pathology
chemical_list: D014408:Biomarkers, Tumor; C507499:KRT20 protein, human; D053554:Keratin-20; D053552:Keratin-7