title: Immunohistochemical expression of PAX5 and TdT by Merkel cell carcinoma and pulmonary small cell carcinoma: a potential diagnostic pitfall but useful discriminatory marker.
abstract: BACKGROUND
  Merkel cell carcinoma is a high-grade neuroendocrine carcinoma of skin that is characterized by immature cells which, because of its striking morphologic similarity, may be confused with other small round blue cell tumors such as pulmonary small cell carcinoma or lymphoblastic leukemia/lymphoma. Immunohistochemistry is therefore paramount to ensuring accurate diagnostic distinction between these tumors. The aim of our study was to evaluate and compare the expression of PAX5 and Terminal deoxynucleotidyl transferase (TdT), in Merkel cell carcinoma and pulmonary small cell carcinoma.
  
  
  METHODS
  PAX5 and TdT immunohistochemical stains were performed on 27 Merkel cell carcinomas and 10 pulmonary small cell carcinomas.
  
  
  RESULTS
  PAX5 was expressed in 24/27 (89%) Merkel cell carcinomas and 0/10 (0%) pulmonary small cell carcinomas. TdT was expressed in 21/27 (78%) Merkel cell carcinomas and 9/10 (90%) pulmonary small cell carcinomas.
  
  
  CONCLUSIONS
  Our study confirms that PAX5 and TdT expression can be expressed in a high percentage of Merkel cell carcinomas and so when positive are not diagnostic of lymphoblastic leukemia/lymphoma. When dealing with metastatic lesions, PAX5 negativity would favor a diagnosis of pulmonary small cell carcinoma over Merkel cell carcinoma. In addition, TTF-1 negative pulmonary small cell carcinoma is to be differentiated from Merkel cell carcinoma.
authors: Department of Pathology, Georgia Health Sciences University Augusta, GA, USA.
authors: Ravindra
authors: R
authors: Kolhe
authors: Michelle D
authors: MD
authors: Reid
authors: Jeffrey R
authors: JR
authors: Lee
authors: Cynthia
authors: C
authors: Cohen
authors: Preetha
authors: P
authors: Ramalingam
mesh_terms: D014408:Biomarkers, Tumor / Q000378:metabolism; D015266:Carcinoma, Merkel Cell / Q000378:metabolism* / Q000473:pathology; D004253:DNA Nucleotidylexotransferase / Q000378:metabolism*; D003937:Diagnosis, Differential; D006801:Humans; D007150:Immunohistochemistry; D008175:Lung Neoplasms / Q000378:metabolism* / Q000473:pathology; D051757:PAX5 Transcription Factor / Q000378:metabolism*; D012878:Skin Neoplasms / Q000378:metabolism* / Q000473:pathology; D055752:Small Cell Lung Carcinoma / Q000378:metabolism* / Q000473:pathology
chemical_list: D014408:Biomarkers, Tumor; D051757:PAX5 Transcription Factor; C497281:PAX5 protein, human; D004253:DNA Nucleotidylexotransferase
keywords: Merkel cell carcinoma; PAX5; TdT; lung; small cell carcinoma