HER2

Some breast cancer cells have a higher than normal level of a protein known as HER2 on their surface, which stimulates them to grow.

Around 20 per cent of breast cancers have this and are called HER2 positive. These cancers tend to grow and spread faster than HER2 negative breast cancers.

HER2 levels

There are three main ways that the pathologist measures HER2 levels.

One is IHC (immunohistochemistry) and this is usually carried out first. This is reported as a score ranging from 0–3.

A score of 0 or 1+ is HER2 negative and means that the breast cancer doesn’t have high levels of HER2.

A score of 2+ is borderline, and a score of 3+ (HER2 positive) means high levels of HER2 have been found.

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 borderline IHC (+2) results should be tested with FISH or CISH to confirm if they are truly HER2 positive. HER2 testing is normally only carried out on invasive breast cancer, so this may not be mentioned if you have ductal carcinoma in situ (DCIS).

Treatment options

People with HER2 positive invasive breast cancer are more 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) but the benefits of others are being looked at in clinical trials. It is likely more targeted therapies will become available in the future.

Only people whose cancer has high levels of HER2 - a protein that makes cancer cells grow - will benefit from having trastuzumab (Herceptin).