Clinical Implications According to Diagnostic Methods of Human Epidermal Growth Factor Receptor 2 Positivity in Breast Cancer: A Retrospective Study

ABSTRACT

PURPOSE: According to American Society of Clinical Oncology/College of American Pathologists guidelines, breast cancer is human epidermal growth factor receptor 2 (HER2) positive if there is HER2 protein overexpression at a 3+ level on immunohistochemistry (IHC 3+) or gene amplification (more than six copies per nucleus) on fluorescence in situ hybridization (FISH+). However, there have been few reports on whether outcomes differ based on diagnosis by these two techniques. In this study, we compared outcomes based on the two methods in patients with HER2-positive breast cancer. METHODS: This study was a retrospective analysis of HER2-positive breast cancer in 18,304 patients, including 14,652 IHC 3+ patients and 3,652 FISH+ patients from the Korean Breast Cancer Society Registry. We compared breast cancer-specific survival and overall survival based on IHC 3+ and FISH+ status with or without trastuzumab. RESULTS: Breast cancer-specific survival was significantly different between the IHC 3+ and FISH+ groups, with 5-year cumulative survival rates of 95.0% for IHC 3+ and 98.5% for FISH+ patients who did not receive trastuzumab (p=0.001) in Kaplan-Meier methods. However, there were no significant differences in breast cancer-specific survival and overall survival between IHC 3+ and FISH+ groups regardless of trastuzumab treatment in Cox proportional hazards models. CONCLUSION: The survival outcomes were not affected by the different two diagnostic methods of HER2-positive breast cancer. Further research to evaluate differences in prognosis and other characteristics according to the diagnostic methods of HER2 positivity is needed in the future.