title: CD99 expression in Merkel cell carcinoma: a case series with an unusual paranuclear dot-like staining pattern.
abstract: BACKGROUND
  Merkel cell carcinoma (MCC) is a rare neuroendocrine cancer of the skin. The utility of CD99 (MIC-2) in the diagnosis of MCC has been previously studied, with reported rates of expression ranging from 13 to 55%. When specified, a membranous or cytoplasmic staining pattern was considered significant. Recent studies of CD99 have identified a paranuclear dot-like expression pattern in certain non-neuroendocrine pancreatic and colonic lesions. We recently noted paranuclear dot-like staining in several cases of MCC, including cases lacking cytokeratin 20 (CK20) expression.
  
  
  METHODS
  Fourteen cases of MCC were stained with CK20 and CD99 antibody, and the pattern and intensity of staining were recorded. Seven cases of pulmonary small cell carcinoma (PSCC) and one case of primitive neuroectodermal tumor (PNET) were used for comparison.
  
  
  RESULTS
  All 14 cases of MCC showed at least focal CD99 staining, with both membranous and paranuclear dot-like staining patterns identified. CK20 staining was present in 12/14 cases, with the characteristic dot-like pattern identified. Four of seven cases of PSCC showed CD99 staining, with two showing a finely granular dot-like staining pattern.
  
  
  CONCLUSIONS
  We report an unusual pattern of paranuclear dot-like expression of CD99 in 14 cases of MCC, two of which did not express CK20. This previously unrecognized expression pattern may be of use in differentiating MCC from other cutaneous malignancies, especially when CK20 expression is limited or absent.
authors.affiliation: Department of Pathology and Molecular Medicine, McMaster University, Hamilton, ON, Canada.
authors.forename: Ashwyn
authors.initials: A
authors.lastname: Rajagopalan
authors.forename: Debra
authors.initials: D
authors.lastname: Browning
authors.forename: Samih
authors.initials: S
authors.lastname: Salama
mesh_terms: D000071443:12E7 Antigen; D000368:Aged; D000369:Aged, 80 and over; D000906:Antibodies / Q000494:pharmacology; D015703:Antigens, CD / Q000276:immunology / Q000378:metabolism*; D014408:Biomarkers, Tumor / Q000378:metabolism; D015266:Carcinoma, Merkel Cell / Q000378:metabolism* / Q000473:pathology*; D015815:Cell Adhesion Molecules / Q000276:immunology / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism / Q000473:pathology; D005260:Female; D006801:Humans; D007150:Immunohistochemistry; D053554:Keratin-20 / Q000276:immunology / Q000378:metabolism; D008175:Lung Neoplasms / Q000378:metabolism / Q000473:pathology; D008297:Male; D008875:Middle Aged; D018242:Neuroectodermal Tumors, Primitive / Q000378:metabolism / Q000473:pathology; D012878:Skin Neoplasms / Q000378:metabolism* / Q000473:pathology*; D055752:Small Cell Lung Carcinoma / Q000378:metabolism / Q000473:pathology
chemical_list: D000071443:12E7 Antigen; D000906:Antibodies; D015703:Antigens, CD; D014408:Biomarkers, Tumor; C061828:CD99 protein, human; D015815:Cell Adhesion Molecules; D053554:Keratin-20