Selecting antibodies to detect HER2 overexpression by immunohistochemistry in invasive mammary carcinomas

There is an increasing clinical demand for HER2 analysis in breast cancer, especially since the release of trastuzumab. The authors assessed the ability of immunohistochemistry to detect HER2 overexpression in invasive mammary carcinomas (IMC) using five antibodies. Paraffin-embedded samples of 86 IMCs (T2N0) were used to compare the immunohistochemical overexpression of HER2 using two polyclonal antibodies (HercepTest [DAKO] and A0485 [DAKO]) and three monoclonal antibodies (CB11 from two different laboratories, Biogenex and Novocastra, and 4D5 [Genentech]). All immunostainings were scored according to the FDA-approved HercepTest recommendations. The Hercep Test-positive cases were compared with gene amplification by FISH (Oncor Inform, Ventana). The HercepTest was positive in 31 of the 86 cases (36.1 %). The DAKO antibody A0485 was positive in 25 of the 66 (37.8%). Monoclonal antibody 4D5 was positive in only 15 of the 86 cases (17.4%). There was almost total agreement in results between the two CB I I antibodies: 25 of the 86 positive cases (29. 1 %). All cases positive for CB I I or 4D5 were HercepTest positive. Most of the HercepTest 2+ cases were negative when using either monoclonal antibody. FISH was positive in 19 of the 20 HercepTest 3+ cases and negative in 5 HercepTest 2+ cases. Three CBII-2+ cases showed no amplification by FISH. In three FISH-positive cases the immunohistochemistry showed no overexpression by all antibodies used. These findings suggest that immunohisto overexpression increases with the use of monoclonal antibodies. The aim of this study was to assess the performance of IHC detection of HER2 overexpression using five antibodies, including the HercepTest, compared with FISH, on archival samples of invasive mammary carcinomas.