Some breast cancer cells have a higher than normal level (known as overexpression) of a protein called HER2 (human epidermal growth factor receptor 2) on their surface, which stimulates them to grow.
Around 15–20% of invasive breast cancers (breast cancer that has the potential to spread to other areas of the body) have this and are called HER2 positive. These cancers tend to grow and spread faster than HER2 negative breast cancers.
All invasive breast cancers are tested for HER2 levels. It’s done in a hospital laboratory on a sample of breast cancer tissue removed during a biopsy or surgery. The results are usually available one to three weeks later.
Outside of a clinical trial, HER2 testing is normally only done on invasive breast cancer, so this is unlikely to be mentioned if you have ductal carcinoma in situ (DCIS). The following are the three most commonly used ways to measure HER2 levels.
IHC (immunohistochemistry) is usually done first. It’s reported as a score ranging from 0–3. A score of 0 or 1+ is means the breast cancer is HER2 negative. A score of 2+ is borderline, and a score of 3+ means the breast cancer is HER2 positive. The other ways of measuring HER2 are called FISH (fluorescent in situ hybridisation) and CISH (chromogenic in situ hybridisation). These are reported as positive or negative. Breast cancers with a borderline IHC result (2+) should be re-tested with FISH or CISH to determine if they are truly HER2 positive.
People with HER2 positive invasive breast cancer are likely to be advised to have chemotherapy and also drug treatments called targeted therapies. These drugs work by blocking specific ways that breast cancer cells divide and grow. The most well-known targeted therapy is trastuzumab (Herceptin). Trastuzumab works by attaching itself to the HER2 proteins (also known as receptors) so that the cancer cells are no longer stimulated to grow. It also helps the body’s immune system destroy breast cancer cells.
Only people whose cancer is HER2+ will benefit from having trastuzumab. People with breast cancer that is HER2 negative will not benefit from trastuzumab.