HER2 testing is primarily used to determine whether a person with breast cancer (and some other cancers, such as certain stomach and oesophageal cancers) may benefit from HER2-targeted treatment such as Herceptin (trastuzumab). In the past, HER2-positive breast cancers were associated with an increased risk of recurrence and a poorer outcome. However, with the use of targeted treatment against HER2 and chemotherapy, the prognosis of HER2-positive breast cancer has improved substantially.

The serum HER2 test is sometimes used to monitor cancer therapy. If the level is initially elevated then falls, it is likely that treatment is working; if it stays elevated, treatment is not working; and if the level falls then rises, the cancer may be recurring.

HER2 testing is recommended as part of an initial work-up of invasive breast cancer and is sometimes done with recurrent breast cancer. It is not diagnostic but helps the doctor determine treatment options and understand more about the tumour’s characteristics.

Serum HER2 is sometimes requested initially to establish a baseline and then, if elevated, used to monitor cancer treatment.

A positive HER2 IHC test means that the HER2 gene is over-expressing (producing an excess amount of) HER2 protein. A positive FISH test means that there is an amplification of the HER2 gene (production of too many copies). If either test is positive, the patient is likely to have an aggressive tumour. HER2-positive patients are potential candidates for HER2 trageted therapy such as Herceptin (trastuzumab).[See "Is there anything else I should know?"].

Currently, Herceptin is approved in Australia for the treatment of HER2 positive early stage breast cancer and HER2 positive advanced breast cancer.

Herceptin use has also been approved in Australia for use in advanced cancers of the stomach or lower oesophagus, if they are shown to be HER2 positive and anti-cancer medications have not yet been commenced.

HER2-positive tumours may be susceptible to Herceptin (trastuzumab), a therapy created to target HER2 protein. Herceptin, an antibody made in the laboratory, attaches itself to the excess protein molecules and inhibits the growth of the cancer. The development of this specialised therapy has increased the use of HER2 testing. Herceptin may be used alone or with some chemotherapy agents but is only useful in those who have HER2 amplification and protein over-expression.

Tissue HER2 testing is not available in every laboratory. Both IHC and FISH require experience and special training to perform and to interpret.

Herceptin use has also been approved in Australia for use in advanced cancers of the stomach, if they are shown to be HER2 positive.

During an initial workup of invasive breast cancer, your doctor is likely to do a tissue test for hormone receptor status. A patient with a positive oestrogen and/or progesterone receptor status may find their response to endocrine or hormone therapy diminished if they are also HER2-positive.

Unfortunately, no. There are other cellular factors involved that are not yet well understood. However Herceptin is often combined with other chemotherapy agents to make it more effective.

Cancers of the stomach and lower oesophagus may be HER2 positive. The use of Herceptin is also approved in Australia in cases of HER2 positive advanced cancer of the stomach, where anti-cancer medications have not yet been commenced.