Understanding HER2 Immunohistochemistry and Scoring

A HER2 score on a pathology report measures how much of one growth-signal protein your tumor carries, read from a stained slice of the biopsy under a microscope. That single number is one of the main things that decides which drugs are offered for breast cancer. The meaning of the bottom of the scale has changed recently, so a tumor once simply called “HER2-negative” may now open a treatment option.1PubMed Central. New treatment options for metastatic HER2-low breast cancer

What HER2 Is and What the Stain Shows

HER2 is a receptor on the surface of cells that helps tell them to grow and divide. In some cancers, extra copies of the HER2 gene lead cells to make far too much HER2 protein.2PubMed Central. The role of HER2 and HER3 in HER2-amplified cancers beyond breast cancers This overproduction turns up in a share of many cancers, including breast, and has been linked to faster-growing disease.3PubMed. The role of overexpressed HER2 in transformation

Immunohistochemistry, or IHC, is the main way HER2 is first checked, looking directly at the protein on the cell membrane.4College of American Pathologists. HER2/ERBB2 In Solid Tumors: A Practical Guide To Testing And IHC Scoring An antibody built to stick only to HER2 is applied to a thin slice of tumor, then a detection system turns each bound spot brown.5Leica Biosystems. Bond Oracle HER2 IHC System Interpretation Guide Breast Tissue The darker and more complete the brown ring around each cell, the more HER2 that cell carries. The test runs on tissue already removed at biopsy or surgery. The pathologist judges both how intense the staining is and what share of cells are stained.

The stain shows how much HER2 protein is present, but it does not directly show the gene, so a separate gene test is sometimes needed. It also says nothing about tumor size, grade or spread.

On your report, look for a line labelled HER2 IHC with a score from 0 to 3+. There may be a separate line for a gene test such as FISH or ISH. Reports often also list estrogen and progesterone receptors alongside HER2.

What Each HER2 Score Means

A few strongly stained cells alone do not make a tumor positive.7Journal of Clinical Oncology. Recommendations for human epidermal growth factor receptor 2 testing in breast cancer: American Society of Clinical Oncology/College of American Pathologists clinical practice guideline update These breast rules, last updated in 2018, were reaffirmed by the main U.S. cancer and pathology societies.8PubMed. Human Epidermal Growth Factor Receptor 2 Testing in Breast Cancer: ASCO-College of American Pathologists Guideline Update

A 3+ result usually leads to a HER2-targeted drug such as trastuzumab, typically given together with chemotherapy rather than alone. Which companion drugs you get and how long treatment lasts depend on the stage of the cancer, so the score opens the option without fixing the whole plan.

The score is not a cancer stage. A 3+ can occur in a small early tumor and a 1+ in a large one. Doctors also read HER2 together with estrogen and progesterone receptor results, and the combination helps name the breast cancer subtype and shape treatment.

What Happens After a 2+ Borderline Result

When IHC is borderline, guidelines call for an automatic second test using a different method.9Journal of Clinical Oncology. Recommendations for human epidermal growth factor receptor 2 testing in breast cancer: American Society of Clinical Oncology/College of American Pathologists clinical practice guideline update A gene test called ISH, often FISH, counts HER2 gene copies in at least 20 cells under fluorescent light.10Journal of Clinical Oncology. Recommendations for human epidermal growth factor receptor 2 testing in breast cancer: American Society of Clinical Oncology/College of American Pathologists clinical practice guideline update The result depends on whether the gene is amplified, judged either by copy number or by its ratio to a marker on the same chromosome.11Journal of Clinical Oncology. Recommendations for human epidermal growth factor receptor 2 testing in breast cancer: American Society of Clinical Oncology/College of American Pathologists clinical practice guideline update This follow-up sorts most borderline IHC cases into a final category.12PubMed. HER2 FISH classification of equivocal HER2 IHC breast cancers with use of the 2013 ASCO/CAP practice guideline

If the gene is amplified, the tumor is called HER2-positive. If it is not, the tumor stays HER2-negative, and a 2+ without amplification is the kind now called HER2-low.

You may get the stain score first and the gene result days later. The gene test normally uses the same tissue sample, so a new biopsy is not usually needed. It is reasonable to ask your oncologist whether treatment planning waits for the final result, since the final status is what guides drug choices.

Sometimes even the gene test is borderline. This is more common when a cell carries extra copies of the whole chromosome HER2 sits on, which raises the gene count without true amplification of HER2 itself.13PubMed. HER2 FISH classification of equivocal HER2 IHC breast cancers with use of the 2013 ASCO/CAP practice guideline Changes to the scoring rules in 2013 also increased both positive and borderline gene results.14PubMed. Updated 2013 College of American Pathologists/American Society of Clinical Oncology (CAP/ASCO) guideline recommendations for human epidermal growth factor receptor 2 (HER2) fluorescent in situ hybridization (FISH) testing increase HER2 positive and HER2 equivocal breast cancer cases; retrospective study of HER2 FISH results of 836 invasive breast cancers In those cases the lab may count more cells, have a second pathologist review, or repeat the testing, so a small number of people wait longer for a final answer.

Why 0 Versus 1+ Now Matters

The drug trastuzumab deruxtecan works not only on HER2-positive tumors but also on HER2-low tumors in metastatic breast cancer.15PubMed Central. New treatment options for metastatic HER2-low breast cancer That means many patients once labelled HER2-negative now have a targeted option.16PubMed Central. New treatment options for metastatic HER2-low breast cancer HER2-low usually means a stain score of 1+, or a 2+ whose gene test showed no amplification. It describes how much HER2 the tumor carries, not a separate type of breast cancer. The catch is that the test was designed to separate high from low HER2, not low from none, so telling 0 from 1+ asks more of it than it was built for.17PubMed Central. HER2 testing: evolution and update for a companion diagnostic assay

“HER2-negative” covers both a score of 0 and HER2-low, and the difference now matters. Before the drug was approved, many reports simply said HER2-negative without stressing whether the score was 0 or 1+. If yours says only negative, the original lab record may still list the exact score, so it is worth asking for it. If the score was never recorded, stored tissue can sometimes be stained and read again.

A few questions are worth bringing to your oncologist:

  • Was my score 0 or 1+? The word negative alone hides the difference.
  • Does HER2-low status change my options now, or only if the cancer spreads? The drug is approved for metastatic disease.
  • Could a sample be rechecked later if the disease spreads, since the first score may be the one on file?

How Reliable HER2 Scoring Is

Clear 3+ and clear 0 results are the most consistent, because strong staining and no staining are easy to tell apart. The weak end is harder. Different pathologists often score the same slide differently.18PubMed Central. Systematic review and meta-analysis of artificial intelligence in classifying HER2 status in breast cancer immunohistochemistry When scoring only HER2-low breast cases, half of pathologist pairs showed no better than chance agreement.19PubMed. Interobserver agreement in scoring HER2-negative and HER2-low immunohistochemistry in breast cancer: Reasons for discordance and impact of a single training session Because 0 versus 1+ now affects treatment eligibility, a small disagreement at that line matters much more than it once did.

Handling matters too. Tissue must be preserved in fixative promptly and for the right length of time. Delays or poor handling before tissue is fixed can break down HER2 protein, dropping a 2+ to 1+ or even 0.20HER2Know. Impact of pre-analytic variables on HER2 IHC results A low score can therefore sometimes reflect how the sample was handled rather than the tumor itself.

Results also vary between laboratories, which is why labs take part in proficiency testing.21PubMed. Standardization of HER2 testing: results of an international proficiency-testing ring study External quality checks and training help labs keep scores consistent,22PubMed Central. Challenges and improvements in HER2 scoring and histologic evaluation: insights from a national proficiency testing scheme for breast cancer diagnosis in China and tailored feedback on lab methods has improved consistency between labs.23PubMed Central. Proficiency testing in immunohistochemistry—experiences from Nordic Immunohistochemical Quality Control (NordiQC)

Computer help is emerging. AI tools can help pathologists score slides, raising accuracy for 0 versus 1+ from about 80% to 93% in one study.24PubMed. The Role of Artificial Intelligence in Accurate Interpretation of HER2 Immunohistochemical Scores 0 and 1+ in Breast Cancer Across studies, AI picked out almost all patients eligible for trastuzumab deruxtecan.25PubMed Central. Systematic review and meta-analysis of artificial intelligence in classifying HER2 status in breast cancer immunohistochemistry They assist the pathologist rather than replace one.

A score sitting right at a cutoff, especially 0 versus 1+, is the most reasonable place to ask for review. You can ask your oncologist to request that the slides be reviewed at another center, which does not require a new biopsy. A result that does not fit the tumor’s other features is also a sound reason to ask for retesting.

Why Scores Can Change Between Samples

HER2 levels can differ from one part of a tumor to another.26PubMed Central. HER2 Intratumoral Heterogeneity in Breast Cancer, an Evolving Concept A needle biopsy takes only a thin core, and if HER2 is patchy, that core may miss the strongest or weakest areas. A small biopsy may therefore give a different score than the full removed tumor.27PubMed Central. HER2 Intratumoral Heterogeneity in Breast Cancer, an Evolving Concept This patchiness is a key reason for borderline results and mismatches between IHC and gene tests.28PubMed Central. HER2 Intratumoral Heterogeneity in Breast Cancer, an Evolving Concept Guidelines advise repeat testing when results seem out of line with other findings,29Journal of Clinical Oncology. Recommendations for human epidermal growth factor receptor 2 testing in breast cancer: American Society of Clinical Oncology/College of American Pathologists clinical practice guideline update so a request to recheck is consistent with standard practice. A surgical specimen, which gives the pathologist much more tissue, may be tested again after a needle biopsy, particularly when the first result was borderline.

When a cancer comes back or spreads, doctors often take a new sample, because the HER2 level in the new tumor can differ from the original. The most recent result usually guides treatment for the current disease. This matters for HER2-low, since a tumor once scored 0 may now score 1+ and become eligible for a drug it was not before. A change in score over time is useful information, not proof that the first test was wrong.

HER2 Testing in Other Cancers

Pathology reporting guidance now covers HER2 IHC in stomach, bowel, gynecological, lung and head and neck cancers.30HER2Know. Updated recommendation to HER2 IHC testing for mNSCLC patients Each organ has its own scoring guidance because tumors there stain differently, so the same brown ring does not carry the same meaning everywhere. Breast and stomach scoring rules differ, and neither has been confirmed for womb cancer.31Elsevier / PubMed Central. Comparison of breast and gastric HER2 immunohistochemistry (IHC) scoring criteria in the assessment of endometrial carcinoma

Stomach tumors often stain unevenly, and the scoring rules allow for that, so a smaller cluster of strongly stained cells can count. Staining may also appear only on part of the cell membrane, and the criteria take that pattern into account.

In spread bowel cancer, HER2 helps guide treatment, and two sets of scoring criteria are in use.32Caris Life Sciences. Evidence for Unified Assessment Criteria of HER2 IHC in Colorectal Carcinoma If your cancer is not breast cancer, ask which scoring system your lab used. HER2 drugs approved for breast cancer may or may not apply to your cancer, so ask your oncologist whether a HER2 result changes your options. If your cancer has spread, ask whether HER2 testing has been done.