Assessment of HER2 status in breast cancer: overall positivity rate and accuracy by fluorescence in situ hybridization and immunohistochemistry in a single institution over 12 years: a quality control study

Background: The gold standard of HER2 status assessment in breast cancer is still debated. Immunohistochemistry (IHC) and in-situ technology as fluorescent-labeled methodology (FISH) can be influenced by pre-analytical factors, assay-conditions and interpretation of test results. We retrospectively conducted this quality control study and analyzed HER2 test results in breast cancer within the routine diagnostic service in a single institution over a period of 12 years. We addressed the question how stable and concordant IHC and FISH methods are and whether HER2 positivity rate has changed over this period. Methods: Data of 7714 consecutive HER2-FISH-assays in a period of 12 years (2001-2012) on breast cancer biopsies and excision specimens were retrospectively analyzed. From 2001 to 2004, FISH tests were performed from all cases with IHC score 3+ and 2+ (and in some tumors with IHC score 1+ and 0). From 2005-2010, HER2 status was only determined by FISH. From 2011-2012, all breast carcinomas were analyzed by both IHC and FISH. Scoring and cut-off-definition were done according to time-current ASCO-CAP and FDA-guidelines. Results: Between 2001-2004, IHC score 3+ was diagnosed in 22% of cases, 69% of these 3+ cases were amplified by FISH. 6% of IHC score 0/1+ cases were amplified by FISH. There was a mean amplification rate of 15.8% (range 13-19%) using FISH only HER2-assays (2005-2010). Starting 2008, a slight drop in the amplification rate from 17% to 14% was noticed due to the modified ASCO-criteria in 2007. From 2011-2012, 12% of cases were 3+ by IHC, 84% of them were amplified by FISH. Less than 1% of IHC score 0/1+ cases were amplified by FISH. Concordance between FISH and IHC increased from 83% to 97%. Conclusions: Our quality control study demonstrates that HER2 positivity rate remained stable by FISH-technology but showed a significant variation by IHC over the analyzed 12 years. Improvement in concordance rate was due to standardization of pre-analytical factors, scoring and interpretation.