HER2 status

What it is

HER2 status is a tumour biomarker result, usually from immunohistochemistry (IHC) and sometimes in situ hybridisation (ISH), that shows whether a cancer has excess HER2 protein or HER2 gene amplification; about 15% to 20% of breast cancers are HER2-positive. HER2 stands for human epidermal growth factor receptor 2, a cell-surface receptor involved in growth signaling. In cancer care, HER2 status matters because it helps predict whether a tumour may respond to HER2-targeted treatment such as trastuzumab, pertuzumab, trastuzumab deruxtecan, or other anti-HER2 therapies.

HER2 testing is routine in newly diagnosed invasive breast cancer and is also relevant in some gastric and gastroesophageal cancers. The result is not the same as cancer stage. Instead, it is a biologic feature of the tumour. A cancer can be HER2-positive, HER2-negative, or sometimes initially equivocal, meaning the first test is not clear and another method is needed.

A simple way to think about it is:

Test result	What it usually means
HER2-positive	The tumour overexpresses HER2 protein and/or has HER2 gene amplification; HER2-targeted therapy may help
HER2-negative	The tumour does not meet criteria for HER2 positivity
Equivocal / borderline	More testing or expert review may be needed

In breast cancer, pathology labs commonly start with IHC, which measures HER2 protein expression on tumour cells. If the IHC result is clearly positive or clearly negative, no further HER2 test may be needed. If it is borderline, ISH is often used to look for extra copies of the HER2 gene.

How it works

HER2 is a receptor tyrosine kinase in the ERBB family. When HER2 signaling is increased, cancer cells may grow, divide, survive, and spread more aggressively. In many HER2-positive tumours, this happens because the ERBB2 gene is amplified, leading to too much HER2 protein on the cell surface.

HER2-targeted drugs work in different ways:

1. Monoclonal antibodies such as trastuzumab bind the HER2 receptor.
2. Dual blockade with drugs such as pertuzumab can further disrupt signaling.
3. Tyrosine kinase inhibitors block signaling inside the cell.
4. Antibody-drug conjugates deliver chemotherapy directly to HER2-expressing cells.

This is why HER2 status is considered a predictive biomarker. It does not just describe the tumour. It helps guide treatment choice.

A newer idea in breast cancer is HER2-low, usually referring to tumours that are not HER2-positive by standard criteria but still show low HER2 expression, often IHC 1+ or 2+ with negative ISH. This category has become clinically relevant because some antibody-drug conjugates may help selected patients with metastatic HER2-low disease. HER2-low is not the same as HER2-positive.

Diagnosis / how it's measured

HER2 status is measured on tumour tissue taken from a biopsy or surgery specimen. The two main methods are:

Method	What it measures	Typical reporting
IHC	HER2 protein on tumour-cell membranes	0, 1+, 2+, 3+
ISH including FISH	HER2 gene amplification in tumour cells	Amplified, not amplified, or occasionally equivocal by detailed criteria

For breast cancer, the common interpretation is:

• IHC 3+: HER2-positive
• IHC 0 or 1+: HER2-negative
• IHC 2+: equivocal, usually needs ISH

Exact interpretation follows current pathology guidelines, which have changed over time. That is one reason older pathology reports may not be directly comparable with newer ones. Pre-analytic factors also matter, including how quickly tissue is fixed after biopsy, how long it is fixed, the quality of the sample, and whether the tumour is heterogeneous, meaning different areas of the tumour show different HER2 patterns.

Sometimes HER2 status is repeated:

1. If the first sample is small or technically limited
2. If the result is borderline or unexpected
3. If cancer recurs or spreads, because biomarker status can differ between the original tumour and a metastasis

In India, HER2 testing is widely used in oncology centres and pathology labs, but test quality can vary by laboratory methods and expertise. For patients, it is reasonable to ask whether the lab follows current ASCO/CAP-style interpretation standards and whether equivocal results are reflexed to ISH.

Evidence and uses

HER2 status is most established in breast cancer, where it is one of the key biomarkers used alongside estrogen receptor, progesterone receptor, grade, and stage. HER2-positive breast cancers historically had a worse prognosis before targeted therapy became available, but outcomes improved substantially after anti-HER2 treatment was introduced.

HER2 testing is also used in gastric and gastroesophageal adenocarcinoma, where HER2 positivity can identify patients who may benefit from HER2-directed treatment. Testing methods and scoring differ somewhat from breast cancer because staining patterns in gastric tumours can be different.

Key clinical uses include:

• Choosing neoadjuvant or adjuvant therapy in early breast cancer
• Selecting treatment in metastatic breast cancer
• Identifying patients for HER2-targeted drugs in gastric or gastroesophageal cancer
• Distinguishing HER2-positive from HER2-low and HER2-zero categories in some advanced breast cancer settings

HER2 status should always be interpreted with the full pathology report. A HER2-positive result does not by itself determine prognosis, and a HER2-negative result does not mean the cancer is mild. Stage, lymph-node involvement, hormone receptor status, tumour grade, and overall health still matter.

When to see a clinician

Discuss HER2 status with your oncologist or pathologist if:

• You have a new diagnosis of invasive breast cancer
• Your report says equivocal, borderline, or pending
• The cancer has recurred or spread and no repeat biopsy has been discussed
• You were told your tumour is HER2-low and want to know whether that changes treatment options
• Your pathology report from one lab differs from another

Bring a copy of the full pathology report, not just a summary line. Ask which test was used, what the exact score was, and whether the result changes treatment. If the result is unclear, a second pathology review at an experienced centre may help.

Limitations and open questions

HER2 testing is highly useful, but it is not perfect. Results can be affected by tissue handling, scoring differences, tumour heterogeneity, and changes in guideline definitions over time. Some tumours sit near the cutoff between categories, which can make interpretation difficult.

Another open question is how best to classify and treat HER2-low and very low HER2 expression. This area is evolving, especially in metastatic breast cancer. Evidence supports some targeted options in selected settings, but HER2-low is not yet equivalent to classic HER2-positive disease.

There is also ongoing research on blood-based HER2 assessment, molecular testing, and how HER2 status may change during treatment or at relapse. For now, tissue-based pathology remains the standard. If a result does not fit the clinical picture, repeat testing or expert review may be appropriate.