Introduction: A New Era for HER2 Testing
The landscape of breast cancer treatment is undergoing a paradigm shift driven by a "new generation" of antibody-drug conjugates (ADCs). These advanced therapies, such as trastuzumab deruxtecan (T-DXd), are proving effective against breast cancers that lack traditional protein overexpression or gene amplification, previously leaving a large patient population without targeted options.
The primary catalyst for this update was the DESTINY-Breast04 trial. Data from this study not only demonstrated a significant survival benefit for patients with low levels of HER2 expression but also prompted the FDA to extend the label indication for T-DXd. This regulatory milestone included the pre-market approval of the Ventana PATHWAY anti-HER2/neu 4B5 monoclonal antibody as a semiquantitative assay to identify patients eligible for this treatment. The 2023 ASCO-CAP guideline update addresses this new clinical reality, focusing on the precision required to identify patients who do not overexpress HER2 but remain candidates for these potent new therapies.
Affirmation of the 2018 Gold Standard
Despite the arrival of new therapeutic indications, the Expert Panel has officially affirmed the 2018 ASCO-CAP recommendations. While the clinical utility of HER2 testing is expanding, the fundamental testing algorithm established in 2018 remains the gold standard for identifying protein overexpression and gene amplification.
To reach this consensus, the panel conducted a systematic review of 173 abstracts. After a rigorous evaluation of the literature, it was determined that no new evidence warranted a revision of the existing testing recommendations or the core interpretive algorithm (Figure 1 in the 2018 guidelines). The 2018 scoring criteria—specifically the distinctions between circumferential and incomplete staining—remain unchanged and must continue to be followed to ensure diagnostic consistency.
The IHC 0 vs. 1+ Threshold: Why It Matters Now
Historically, the distinction between an Immunohistochemistry (IHC) score of 0 and 1+ was clinically negligible, as both results were categorized as "HER2-negative." Today, this threshold has become "artifactually" critical. It is important to note that this divide is a result of clinical trial design rather than a proven biological limit; specifically, patients with IHC 0 results were excluded from the DESTINY-Breast04 trial, making IHC 1+ the current regulatory floor for treatment eligibility.
Clinical Implications of IHC Scores
| HER2 IHC/ISH Status | Clinical Classification | Eligibility for Targeted Therapy |
|---|---|---|
| IHC 3+ or IHC 2+/ISH Amplified | HER2-Positive | Eligible for conventional HER2-targeted therapies (e.g., trastuzumab, pertuzumab, T-DM1). |
| IHC 1+ or IHC 2+/ISH Not-Amplified | HER2-negative for protein overexpression/gene amplification | Eligible for trastuzumab deruxtecan based on non-overexpressed protein levels. |
| IHC 0 | HER2-Negative (No staining or minimal/incomplete) | Currently ineligible for newer HER2-targeted ADCs based on trial entry criteria. |
Addressing the "HER2-Low" Terminology
While the term "HER2-Low" was utilized as shorthand during the DESTINY-Breast04 trial to describe the IHC 1+ and IHC 2+/ISH not-amplified populations, the Expert Panel considers it premature to adopt this or "HER2-Ultra-Low" as formal diagnostic categories.
The panel cited several biological and technical reasons for this caution:
- Reporting Delays for IHC 2+: Adopting a "HER2-Low" category would complicate the reporting of IHC 2+ cases. Because these cases are equivocal for protein overexpression, their final status (Low vs. Positive) cannot be determined until reflex ISH results are available, which would cause significant reporting delays.
- Instability of Status: Research indicates that the distinction between IHC 0 and "Low" is unstable. Approximately 40% of cases switch categories when comparing paired primary and metastatic samples, making it a poor foundation for a distinct biological subtype.
- Technical Sensitivity: Current IHC assays were optimized to detect high-level overexpression. They may be suboptimal for quantifying the lower limits of protein expression, where preanalytic factors (fixation and processing) have a disproportionate impact on results.
In short, "HER2-Low" describes a specific trial population rather than a reproducibly defined biological entity.
Clinical Imperatives for Accuracy
To ensure patients are correctly identified for ADC therapy, the panel emphasizes maximum precision in distinguishing IHC 0 from 1+.
Pathology Best Practices:
- High-Power Examination: Pathologists must use 40x magnification when discriminating between IHC 0 and 1+ staining patterns.
- Standardized Controls: Laboratories should utilize controls representing a range of protein expression (including 1+) to confirm the assay’s limit of detection.
- Second Review: A second pathologist review is strongly recommended for borderline cases, particularly those showing faint, barely perceptible, incomplete membrane staining in more than 10% of cells.
- Standardized Scoring: Adhere strictly to the 2018 definitions for staining intensity and percentage of tumor cells.
Medical Oncologist Action Item: Oncologists should review HER2 IHC results from all available tissue, including prior primary samples and multiple metastatic sites. Due to the 40% switch rate between paired samples and the potential for preanalytic issues in metastatic biopsies, a patient may be identified as eligible in one sample even if another tests as IHC 0.
Standardizing the Report: The New Recommended Footnote
To ensure clear communication, it is now a clinical requirement to report the specific semiquantitative IHC score (0, 1+, 2+) alongside the "HER2-negative" designation. Pathologists should include the following recommended footnote in HER2 testing reports:
"Patients with breast cancers that are HER2 IHC 3+ or IHC 2+/ISH amplified may be eligible for several therapies that disrupt HER2 signaling pathways. Invasive breast cancers that test 'HER2-negative' (IHC 0, 1+ or 2+/ISH not-amplified) are more specifically considered 'HER2-negative for protein overexpression/gene amplification' since non-overexpressed levels of the HER2 protein may be present in these cases. Patients with breast cancers that are HER2 IHC 1+ or IHC 2+/ISH not-amplified may be eligible for a treatment that targets non-amplified/non-overexpressed levels of HER2 expression for cytotoxic drug delivery (IHC 0 results do not result in eligibility currently)."
Conclusion: Key Clinical Takeaways
As HER2 characterization becomes increasingly nuanced, clinicians must integrate these three takeaways:
- Guideline Continuity: The 2018 ASCO-CAP standards remain the foundation for all HER2 testing and should be followed without modification to the core algorithm.
- Precision in Reporting: The numerical IHC score (0 vs. 1+) is now a critical gateway for treatment. Pathologists must report the specific score (e.g., "HER2-negative for protein overexpression (1+ staining present)").
- Clinical Context: Results must be interpreted within the context of specific drug approvals. While data from the DAISY trial (a single-arm phase II study reported in abstract form) suggest that IHC 0 patients might also respond to these therapies, current regulatory eligibility is strictly tied to the IHC 1+ or 2+/ISH-negative threshold.
Looking ahead, the development of more sensitive assays and the completion of ongoing clinical trials will be essential to determine if the IHC 0 population may eventually benefit from this new generation of ADCs.