Breast Cancers With Discordant Human Epidermal Growth Factor Receptor 2 Fluorescence In Situ Hybridization–Positive (Group 1) and Immunohistochemistry-Negative (0/1+) Results Following American Society of Clinical Oncology/College of American Pathologists Guideline

Key Points

• To investigate the differences in HER2 status between discordant and concordant breast cancers, especially focusing on testing methods.
• Analyzed breast cancers using dual immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH) testing.
• Emphasized the importance of testing from metastatic sites for accurate eligibility for therapies.
• Examined HER2 copy number discrepancies between cases.
• Found significant differences in HER2 copy numbers between discordant and concordant cases.
• Identified a potential need for more thorough testing to uncover biological differences.