Evaluating HER2 amplification and overexpression in breast cancer

The development of Herceptin (Trazumatab) makes testing for HER2 status imp ortant for choosing optimal therapy in breast cancer. This study addresses the precision, accuracy, and reproducibility of HER2 assays. HER2 was asses sed retrospectively by immunohistochemistry (IHC) with Dako 'Herceptest', b y IHC with the monoclonal antibody CB11, and by fluorescence in situ hybrid ization (FISH, PathVysion), in a series of 216 formalin-fixed breast carcin omas including 191 for which quantitative HER2 data from radioimmunohistoch emistry (Q-IHC) were available. All tests were scored independently by two observers. Positivity rates varied between Herceptest (12.6%), FISH (19.4%) , and CB11 IHC (28.5%). Kappa values showed that IHC-based tests were more susceptible to inter-observer variation (kappa = 0.67 and 0.74 for Hercepte st and CB11, respectively) than FISH (kappa = 0.973). Overall test accuracy (see the Materials and methods section) for CB11 IHC (83.8%) was lower tha n Herceptest (87.4%) or FISH (93.2%,,). FISH predicted p185 HER2 overexpres sion (determined by Q-IHC) better (concordance index C.Ind. 0.90) than CB11 IHC (C.Ind. = 0.85) or Herceptest (C.Ind. = 0.81). Of 42 cases with gene a mplification by FISH, 67%, were positive in the Herceptest (2 + or 3 +) vs. 83% with CB11. Of 174 cases negative by FISH, 96%, were negative in the He rceptest and 68% with CB11. conclusion, FISH is the most accurate, reproduc ible, and precise predictor of HER2 overexpression in routine diagnostic la boratories. Copyright (C) 2001 John Wiley & Sons, Ltd.