title: HER2 testing of multifocal invasive breast carcinoma: how many blocks are enough?
abstract: OBJECTIVE
  To determine the diagnostic yield of testing multiple blocks for HER2 in cases of multifocal breast carcinoma.
  
  
  METHODS
  We identified 246 consecutive cases of multifocal invasive breast carcinoma in which HER2 was tested on more than 1 tumor focus. We performed an audit of all cases with respect to tumor size, grade, and histologic type.
  
  
  RESULTS
  HER2 status was concordant between multiple foci in 230 (93.5%) of 246 cases, with the largest focus having the most positive HER2 result in 242 (98.4%) of 246 cases. We did not find a single case in which a smaller focus demonstrated a more positive HER2 status unless this focus was either higher grade or different histologically.
  
  
  CONCLUSIONS
  Our findings support the evaluation of HER2 on the largest focus, with additional testing on smaller foci having a different histologic type or higher grade.
authors.affiliation: Dept of Pathology and Laboratory Medicine, Mount Sinai Hospital, 600 University Ave, 6-500, Toronto, ON M5G 1X5, Canada; e-mail: mchang2@mtsinai.on.ca.
authors.forename: Gillian C
authors.initials: GC
authors.lastname: Bethune
authors.forename: J Brendan
authors.initials: JB
authors.lastname: Mullen
authors.forename: Martin C
authors.initials: MC
authors.lastname: Chang
mesh_terms: D014408:Biomarkers, Tumor / Q000235:genetics / Q000378:metabolism; D001943:Breast Neoplasms / Q000175:diagnosis* / Q000235:genetics / Q000378:metabolism; D018270:Carcinoma, Ductal, Breast / Q000175:diagnosis* / Q000235:genetics / Q000378:metabolism; D005260:Female; D006801:Humans; D017404:In Situ Hybridization, Fluorescence / Q000379:methods / Q000592:standards; D009378:Neoplasms, Multiple Primary; D017410:Practice Guidelines as Topic; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000235:genetics / Q000378:metabolism*; D013048:Specimen Handling / Q000379:methods* / Q000706:statistics & numerical data
chemical_list: D014408:Biomarkers, Tumor; D018719:Erb-b2 Receptor Tyrosine Kinases; C508053:ERBB2 protein, human
keywords: Breast carcinoma; FISH; HER2; Immunohistochemistry; Multifocal