Analysis of molecular subtypes for the increased HER2 equivocal cases caused by application of the updated 2013 ASCO/CAP HER2 testing guidelines in breast cancer

Accurate testing of the status of human epidermal growth factor receptor type 2 (HER2) is a prerequisite for HER2-directed therapy. The American Society of Clinical Oncology (ASCO) and the College of American Pathologists (CAP) published joint guideline recommendations for HER2 testing in breast cancer in 2007 and it was updated in 2013. We compared the HER2 gene amplification status based on these two guidelines and analyzed the molecular characteristics of the equivocal cases.

A total of 1894 patient samples were analyzed for both immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH). HER2 FISH amplification was examined and re-assessed using 2013 guidelines.

According to the 2013 ASCO/CAP recommendations, 763 (40.3%) cases were classified as HER2 positive compared with 729 (38.5%) cases defined by 2007 guidelines. There was a significant increase of 6.1% in the proportion of HER2 FISH equivocal cases that were interpreted using ASCO/CAP 2013 (7.3%) compared with 2007 (1.2%) guidelines (P < 0.001). Of 138 FISH equivocal cases defined by 2013 guidelines, 125 cases were IHC2+ and 13 cases were IHC1+. These 125 cases included 4 double equivocal cases which were defined as equivocal by both 2007 and 2013 guidelines and 121 cases whose status was changed from negative defined by 2007 guidelines to equivocal defined by 2013 guidelines. Compared with luminal A type and luminal B type respectively, these 121 equivocal cases demonstrated no significant difference with luminal B type in T stage and N stage (P = 0.192, P = 0.421). When we divided the luminal B type into two parts that included HER2 negative cases and HER2 positive cases, the equivocal cases also showed no significant difference with these two subtypes in T stage and N stage.

Our study suggested that implementation of the revised ASCO/CAP 2013 guidelines resulted in an increase of 1.7% in overall HER2 positivity rate and of 6.1% in equivocal cases. Pathological analysis revealed that these equivocal cases exhibit similar biological behavior with luminal B type tumors. Clinical utility data on targeted therapy in equivocal patients should be further investigated.