HER2 fluorescence in situ hybridization groups 2-4 breast cancers classified as positive after targeted recounts following equivocal (2+) immunohistochemistry

Objectives: To investigate the correlation between the extent of (percentage of tumor cells) immunohistochemistry (IHC) staining and final human epidermal growth factor receptor 2 (HER2)-positive result in fluorescence in situ hybridization (FISH) groups 2 to 4 with equivocal (2+) IHC requiring second, blinded FISH evaluation. Methods: Breast cancer cases submitted for HER2 FISH testing with group 2 to 4 results were included. Results: Of the 2548 cases with HER2 FISH groups 2 to 4 that had HER2 IHC performed, 1104 (43.3%) (76/182 [41.8%] of group 2, 94/161 [58.4%] of group 3, 934/2205 [42.4%] of group 4) had equivocal (2+) IHC. After second blinded, IHC-guided recounts, 217 of 1104 (19.7%) (17/76 [22.4%], 75/94 [79.8%], 125/934 [13.4%] of FISH groups 2, 3, 4 with IHC 2+, respectively) had final HER2-positive status. Only 13 of 217 (6%) of the cases with HER2-positive status had more than 50% circumferential staining of the tumor targeted for rescoring. Conclusions: In over 90% of HER2 FISH group 2 to 4 breast cancers with equivocal (2+) IHC followed by targeted, blinded second FISH evaluation and final HER2-positive result, the amplified population of tumor cells was limited (<50%). Current guidelines recommend cancers having 10% to 50% of the subpopulation with amplified cells classified as having genetic heterogeneity (GH), which have a poor response to targeted therapies. Identifying these tumors as having GH and/or repeat testing may be recommended.