title: Expression of melanoma antigens in epithelioid gastrointestinal stromal tumors: a potential diagnostic pitfall.
abstract: BACKGROUND
  Most gastric gastrointestinal stromal tumors (GISTs) express CD117/c-kit, as do a subset of metastatic melanomas, leading to a diagnostic dilemma in some cases.
  
  
  OBJECTIVE
  To further differentiate GISTs from melanoma, we investigated expression of melanoma markers in GISTs using a well-characterized set of gastric lesions on tissue microarrays.
  
  
  METHODS
  Tissue microarrays from paraffin-embedded tissue cores from 38 patients were stained with S100 protein, HMB-45, and Melan-A antibodies. All cases had been previously stained with CD117/c-kit and CD34 antibodies. All were reactive with CD117/c-kit, and 88.2% expressed CD34.
  
  
  RESULTS
  S100 protein was focally expressed in 2 (5.3%) of 38 GISTs; these lesions lacked HMB-45 and Melan-A labeling. No tumor labeled with HMB-45, but 4 (10.6%) of 38 cases labeled with Melan-A antibodies. The Melan-A-reactive cases were all S100 negative and CD34 positive. The S100-reactive cases were spindle cell neoplasms, whereas the Melan-A-reactive cases were epithelioid neoplasms (4/9; 44%). An additional 15 standard sections of separate cases of epithelioid GISTs were then labeled with Melan-A, and 5 (33%) of 15 showed at least focal labeling.
  
  
  CONCLUSIONS
  Melan-A staining can be encountered in a subset of epithelioid GISTs, a finding that can suggest a differential diagnosis of melanoma. In this series, the Melan-A-reactive cases lacked S100 protein and expressed CD34, both of which would be unlikely in melanoma. As such, a panel approach is best in differentiating epithelioid GISTs from melanoma.
authors.affiliation: Department of Pathology, Johns Hopkins University, Baltimore, MD 21231, USA.
authors.forename: Mehmet L
authors.initials: ML
authors.lastname: Guler
authors.forename: Jason A
authors.initials: JA
authors.lastname: Daniels
authors.forename: Susan C
authors.initials: SC
authors.lastname: Abraham
authors.forename: Elizabeth A
authors.initials: EA
authors.lastname: Montgomery
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D018952:Antigens, CD34 / Q000032:analysis; D000951:Antigens, Neoplasm / Q000032:analysis*; D014408:Biomarkers, Tumor / Q000032:analysis; D003937:Diagnosis, Differential; D005260:Female; D046152:Gastrointestinal Stromal Tumors / Q000175:diagnosis* / Q000276:immunology*; D006801:Humans; D058965:MART-1 Antigen; D008297:Male; D008545:Melanoma / Q000175:diagnosis*; D058950:Melanoma-Specific Antigens; D046228:Microarray Analysis; D008875:Middle Aged; D009363:Neoplasm Proteins / Q000032:analysis*; D009418:S100 Proteins / Q000032:analysis
chemical_list: D018952:Antigens, CD34; D000951:Antigens, Neoplasm; D014408:Biomarkers, Tumor; D058965:MART-1 Antigen; C548667:MLANA protein, human; D058950:Melanoma-Specific Antigens; D009363:Neoplasm Proteins; D009418:S100 Proteins