Why do cancers come back?

Sometimes cancer can come back. This can happen because tiny cancer cells, which can’t be seen with the naked eye or on scans, can be left behind after cancer treatment.

Over time these cancer cells can begin to divide again and form a tumour.

Treatment may be given to try to get rid of all the cancer so that it doesn’t come back. Many people may have an operation to remove the tumour. Often, to make sure all the cancer cells are taken away during the operation, some healthy tissue around the tumour will also be removed. To help reduce the risk of any cancer cells being left behind after surgery, other treatments such as chemotherapy, radiotherapy andbiological therapy may also be used.

For some types of cancer an operation isn’t an appropriate treatment. In this case chemotherapy, radiotherapy or biological therapy may be used. These treatments aim to destroy as many of the cancer cells as possible. Often a combination of treatments is given, which can be more effective.

Unfortunately, no treatment is guaranteed to be 100% effective. Sometimes cancer cells can remain and in some people the cancer might come back - sometimes many years later.

If cancer comes back in the same area of the body it’s known as a local recurrence. If cancer develops in a different part of the body, it’s called a metastasis or secondary cancer. A secondary cancer can develop if cancer cells break away from the original (primary) cancer and spread to other organs in the body through the bloodstream or lymphatic system. When these cells reach a new area of the body they grow to form a new tumour.

If cancer does come back it can often be treated again. Usually in this situation, treatment is given to control the cancer, but it may be possible to give treatment that aims to get rid of the cancer.