Human Epidermal Growth Factor Receptor 2 Testing in Breast Cancer.

Area of Science:

• Oncology
• Pathology
• Genetics

Background:

• Current human epidermal growth factor receptor 2 (HER2) testing guidelines focus on identifying overexpression or amplification for targeted therapies.
• A new generation of HER2-targeting antibody-drug conjugates shows efficacy in cancers with lower HER2 expression.
• The need to update recommendations arises from new therapeutic indications for HER2-low breast cancer.

Approach:

• A systematic literature review was conducted to identify signals for updating existing recommendations.
• The panel reviewed 5 potential publications identified from 173 abstracts.
• No signals were found to revise the current 2018 ASCO-CAP recommendations.

Key Points:

• The 2018 ASCO-CAP recommendations for HER2 testing in breast cancer remain affirmed.
• Trastuzumab deruxtecan is indicated for HER2-positive breast cancers with IHC 1+ or 2+ without amplification.
• Distinguishing between IHC 0 and IHC 1+ is clinically relevant for emerging therapies, though new categories are premature.

Conclusions:

• Existing ASCO-CAP guidelines for HER2 testing are reaffirmed.
• Best practices for differentiating IHC 0 from IHC 1+ are now clinically relevant due to new antibody-drug conjugate approvals.
• While new result categories like HER2-Low are not yet supported, precise reporting of IHC 0 versus 1+ is crucial.