# Study Guide: ASCO–CAP 2023 Guideline Update on HER2 Testing in Breast Cancer This study guide provides a comprehensive overview of the 2023 update to the American Society of Clinical Oncology (ASCO) and the College of American Pathologists (CAP) clinical practice guidelines for Human Epidermal Growth Factor Receptor 2 (HER2) testing in breast cancer. --- ## 1. Executive Summary and Purpose The 2023 Update Panel conducted a systematic literature review to determine if existing HER2 testing recommendations required revision. The primary impetus for this review was the emergence of new antibody-drug conjugates (ADCs), specifically **trastuzumab deruxtecan**, which showed efficacy in patients previously classified as "HER2-negative." ### Key Findings * **Affirmation of 2018 Guidelines:** The Update Panel affirmed the 2018 ASCO–CAP recommendations for HER2 testing. No evidence supported a change to the fundamental scoring or reporting categories. * **Identification of HER2-Low:** While "HER2-Low" (IHC 1+ or IHC 2+/ISH not-amplified) is not yet a formal regulatory category, distinguishing IHC 1+ from IHC 0 is now clinically relevant for determining eligibility for trastuzumab deruxtecan. * **Artifactual Threshold:** The threshold between IHC 0 and IHC 1+ was created by trial entry criteria (DESTINY-Breast04) rather than a demonstrated biological or prognostic difference. --- ## 2. HER2 Immunohistochemistry (IHC) Scoring Algorithm The following table outlines the standardized 2018 algorithm for evaluation of HER2 protein expression, which remains the current standard for clinical practice. | IHC Score | Result | Staining Pattern (Invasive Component) | | :--- | :--- | :--- | | **3+** | **Positive** | Circumferential membrane staining that is complete, intense, and in >10% of tumor cells. | | **2+** | **Equivocal** | Weak to moderate complete membrane staining observed in >10% of tumor cells. Requires reflex ISH testing. | | **1+** | **Negative** | Incomplete membrane staining that is faint/barely perceptible and in >10% of tumor cells. | | **0** | **Negative** | No staining observed **OR** membrane staining that is incomplete and faint/barely perceptible in ≤10% of tumor cells. | ### Reflex Testing Requirements For cases scored as **IHC 2+ (Equivocal)**, the pathologist must: 1. Order a reflex test on the same specimen using In Situ Hybridization (ISH). 2. Alternatively, order a new test on a new specimen (if available) using IHC or ISH. --- ## 3. Best Practices for Distinguishing IHC 0 from IHC 1+ Because eligibility for certain newer therapies hinges on the distinction between IHC 0 and IHC 1+, the Update Panel recommends the following best practices: * **High-Power Examination:** Use 40x magnification to discriminate between score 0 and score 1+. * **Standardized Criteria:** Adhere strictly to ASCO–CAP guideline scoring criteria. * **Second Pathologist Review:** Consider a secondary review when results are near the 0 vs. 1+ threshold (e.g., cases with >10% of cells showing faint, barely perceptible staining). * **Control Range:** Use controls with a range of protein expression (including IHC 1+) to ensure the assay’s limit of detection is appropriate. * **Sample Consideration:** Oncologists should review HER2 results from prior primary or other metastatic sites, as expression can be heterogeneous or impacted by preanalytic conditions. --- ## 4. Reporting Requirements Pathology laboratories are required to report the semiquantitative IHC score (0, 1+, 2+, 3+) rather than just a "Positive" or "Negative" result. ### Mandatory Reporting Footnote Laboratories should include a footnote in HER2 IHC and ISH reports with the following language: > "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)." --- ## 5. Tissue Handling and Preanalytics To ensure accurate HER2 results, the following preanalytic standards must be met: * **Fixative:** 10% neutral buffered formalin. * **Fixation Duration:** 6 to 72 hours. * **Specimen Slicing:** Samples should be sliced at 5- to 10-mm intervals. * **Time to Fixation:** Should be as short as possible. * **Section Age:** Sections cut >6 weeks earlier may not be suitable for testing depending on storage and primary fixation. --- ## 6. Short-Answer Practice Questions 1. **Why did the Update Panel decide to affirm the 2018 recommendations rather than creating a new "HER2-Low" category?** * *Answer:* There is currently a lack of evidence that "HER2-Low" is a reproducibly defined subtype with distinct prognostic or predictive implications. Furthermore, the status appears unstable across patient samples (40% switch status between primary and metastatic samples). 2. **Which clinical trial served as the impetus for revisiting the HER2 guidelines in 2022?** * *Answer:* The DESTINY-Breast04 trial. 3. **What is the defining characteristic of an IHC 3+ positive result?** * *Answer:* Intense, complete circumferential membrane staining in more than 10% of tumor cells. 4. **How should a pathologist handle a specimen that shows strong membrane staining of normal breast ducts?** * *Answer:* The IHC test should be rejected and repeated (or tested by FISH) because this internal control indicates an analytical issue. 5. **What magnification is specifically recommended for discriminating between IHC 0 and IHC 1+?** * *Answer:* High power (40x). 6. **Under what condition is a HER2 test reported as "Indeterminate"?** * *Answer:* When technical issues prevent reporting (e.g., inadequate specimen handling, artifacts like crush/edge effects, or analytical testing failure). --- ## 7. Essay Prompts for Deeper Exploration 1. **The "Artifactual Threshold" Controversy:** Discuss the implications of basing drug eligibility on the IHC 0 versus IHC 1+ threshold. Include in your discussion the findings from the DESTINY-Breast04 trial regarding IHC 0 exclusion and the DAISY trial findings regarding IHC 0 response. 2. **Evolution of HER2 Targeted Therapy:** Trace the expansion of HER2-targeted agents from the original trastuzumab to the current generation of ADCs. How has the goal of HER2 testing shifted from identifying "signaling disruption" candidates to "cytotoxic drug delivery" candidates? 3. **Challenges in Pathological Interpretation:** Analyze why the distinction between IHC 0 and IHC 1+ is more difficult for pathologists than identifying IHC 3+. Discuss the role of preanalytic factors and the limitations of current semiquantitative IHC assays in detecting low levels of protein. --- ## 8. Glossary of Important Terms * **ADC (Antibody-Drug Conjugate):** A class of drugs (e.g., trastuzumab deruxtecan) that uses an antibody to deliver a cytotoxic chemical "payload" specifically to cells expressing a target protein, even at non-overexpressed levels. * **Artifactual Threshold:** A clinical boundary (such as IHC 0 vs. 1+) created by the specific design and entry criteria of a clinical trial rather than a natural biological division. * **CEP17 (Chromosome Enumeration Probe 17):** A probe used during ISH testing to determine the ratio of HER2 genes to the number of copies of chromosome 17. * **IHC (Immunohistochemistry):** A testing method that uses antibodies to detect specific proteins (HER2) in a tissue sample to determine the level of protein expression on the cell membrane. * **ISH (In Situ Hybridization):** A testing method used to detect the number of copies of the HER2 gene in a cell to determine if gene amplification is present. * **Preanalytic Conditions:** Factors occurring before the actual laboratory test, such as the time to fixation, the type of fixative used, and the duration of fixation, which can significantly affect the results of HER2 testing. * **Reflex Testing:** An automatic secondary test (usually ISH) performed when the initial test result (usually IHC) is equivocal (2+), intended to provide a definitive positive or negative result. * **Trastuzumab Deruxtecan:** A HER2-targeted ADC shown to be effective in breast cancers with low HER2 expression (IHC 1+ or IHC 2+/ISH negative).