title: Utility of CD99 Paranuclear Expression in the Differential Diagnosis of Merkel Cell Carcinoma.
abstract: BACKGROUND
  Recent reviews have referred to the paranuclear dot-like staining pattern of CD99 in several neoplasms, including solid pseudopapillary tumors in the pancreas, colonic adenocarcinomas, and colonic adenomas as well as in Merkel cell carcinoma (MCC). The aim of this work was to explore the utility of CD99 paranuclear staining in the differential diagnosis of MCC.
  
  
  METHODS
  We explore paranuclear dot-like CD99 expression in several small, round blue cell neoplasms, including neuroendocrine neoplasms, Ewing sarcomas/primitive neuroectodermal tumors (EWS/PNET), melanomas, small cell lung carcinomas (SCC), lymphoblastic lymphoma/leukemia, and rhabdomyosarcomas, in comparison with 33 cases of MCC, to determine the specificity of the paranuclear dot-like CD99 expression in MCC.
  
  
  RESULTS
  Twenty MCC (60%) demonstrated focal expression of CD99 and of those, 14 (42.4%) showed the characteristic paranuclear dot-like expression. CD99 was also paranuclear positive in 4 of 11 (36%) SCC, in 3 of 7 (43%) EWS/PNET, in 1 of 6 (16%) lymphoblastic lymphoma/leukemia cases, in 3 of 3 (100%) rhabdomyosarcomas and all melanomas were negative for the CD99 reaction.
  
  
  CONCLUSIONS
  CD99 paranuclear dot-like expression was not exclusive of the MCC compared with several neoplasms included in its differential diagnosis. This expression is not a great diagnostic aid.
authors.affiliation: Instituto Nacional de Cancerología, Mexico DF, Mexico.
authors.forename: Hugo R
authors.initials: HR
authors.lastname: Domínguez-Malagón
authors.affiliation: Charles University Hospital Pilsen, Pilsen, Czech Republic.
authors.forename: Michal
authors.initials: M
authors.lastname: Michal
authors.affiliation: Charles University Hospital Pilsen, Pilsen, Czech Republic.
authors.forename: Dmitry V
authors.initials: DV
authors.lastname: Kazakov
authors.affiliation: Instituto Nacional de Cancerología, Mexico DF, Mexico.
authors.forename: Claudia H
authors.initials: CH
authors.lastname: Caro-Sánchez
authors.affiliation: Instituto Nacional de Cancerología, Mexico DF, Mexico saul.lino.sil@gmail.com.
authors.forename: Leonardo S
authors.initials: LS
authors.lastname: Lino-Silva
mesh_terms: D000071443:12E7 Antigen / Q000032:analysis / Q000096:biosynthesis*; D014408:Biomarkers, Tumor / Q000032:analysis*; D015266:Carcinoma, Merkel Cell / Q000175:diagnosis* / Q000378:metabolism; D003937:Diagnosis, Differential; D006801:Humans; D007150:Immunohistochemistry; D012878:Skin Neoplasms / Q000175:diagnosis* / Q000378:metabolism
chemical_list: D000071443:12E7 Antigen; D014408:Biomarkers, Tumor; C061828:CD99 protein, human
keywords: CD99; Merkel; immunohistochemistry