Assessment of discordance of immunohistochemistry and in SITU hybridization HER2 testing results in breast cancer specimens: A regional slide-exchange ring study

Abstract

Objective: A regional slide-exchange ring study with four testing rounds was designed in order to assess immunohistochemistry (IHC) and in situ hybridization (ISH) interlaboratory consensus of HER2-testing in breast cancer among four experienced testing centers. Method: In each round, one center selected and sent to the other centers two specimen sets (one for IHC, and one for ISH) consisting of four breast cancers. Institutions could participate irrespective of the staining methods, protocols, and antibodies used for IHC or ISH testing of HER2 status. Results were analyzed by an independent coordinator. Results: After the IHC testing complete consensus among the four institutions was achieved for 7 (43.8 %) of the 16 specimens. Retesting by ISH of the 8 specimens scored as equivocal by at least one laboratory, increased the concordance rate to 68.8 % (11/16), while a diagnostic discordance was found in two cases. A complete concordance between testing centers for ISH was found for 9 (56.3 %) of the 16 specimens. The group of discordant findings consisted of 4 cases having borderline HER2-positivity determined by at least one center, and one case with diagnostic discordance. Conclusion: The current regional ring study in which the level of consensus among the testing centers for HER2 testing by both IHC and ISH was similar to already reported in previous studies, in spite of the unstandardized conditions, highlights the usefulness of implementing such slide-exchange programs as an additional instrument for external quality control.