title: Distinguishing carcinoid tumor of the mediastinum from thymoma: correlating cytologic features and performance in the College of American Pathologists Interlaboratory Comparison Program in Nongynecologic Cytopathology.
abstract: BACKGROUND
  The cytologic features of carcinoid tumor in mediastinal fine-needle aspiration are well described. Nevertheless, this tumor may be difficult to distinguish from thymoma in this site.
  
  
  OBJECTIVE
  We sought to correlate the cytologic features of carcinoid tumor of the mediastinum in the College of American Pathologists Interlaboratory Comparison Program in Nongynecologic Cytopathology with the frequency of misclassification as thymoma.
  
  
  METHODS
  We reviewed 446 interpretations from 18 different cases of carcinoid tumor in mediastinum and correlated the cytologic features with performance.
  
  
  RESULTS
  Cases were more frequently classified as thymoma (158 responses, 35%) than as carcinoid tumor (126 responses, 28%). The best-performing case was classified as carcinoid tumor only 56% of the time. Three cytologic patterns were identified. Four cases consisted of isolated round cells with salt-and-pepper chromatin. Four cases consisted of isolated spindle and round cells with salt-and-pepper chromatin. The remaining 10 cases consisted of cohesive fragments of crowded cells with finely granular chromatin showing numerous pyknotic cells mimicking lymphocytes. Prominent vasculature patterns were not a feature of any of the cases. There was no correlation between any pattern and the rate of classification as carcinoid tumor or thymoma (P > .05).
  
  
  CONCLUSIONS
  Carcinoid tumor of the mediastinum is frequently misclassified as thymoma in this program. Although some cytologic patterns resemble thymoma, the lack of correlation of these patterns with performance suggests that at least part of the reason for misclassification may be failure to consider the correct diagnosis or a lack of familiarity with discriminating cytologic criteria.
authors.affiliation: Department of Pathology, Baptist Hospital of Miami, Miami, Fla, USA.
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Renshaw
authors.forename: Jennifer C
authors.initials: JC
authors.lastname: Haja
authors.forename: Margaret H
authors.initials: MH
authors.lastname: Neal
authors.forename: David C
authors.initials: DC
authors.lastname: Wilbur
mesh_terms: D044963:Biopsy, Fine-Needle; D002276:Carcinoid Tumor / Q000473:pathology*; D002843:Chromatin / Q000648:ultrastructure; D003937:Diagnosis, Differential; D003951:Diagnostic Errors; D006801:Humans; D007753:Laboratories; D008479:Mediastinal Neoplasms / Q000473:pathology*; D009656:North America; D010340:Pathology, Surgical; D012955:Societies, Medical; D013945:Thymoma / Q000473:pathology*; D013953:Thymus Neoplasms / Q000473:pathology*
chemical_list: D002843:Chromatin