The short answer to this question is that when a cancer 'comes back' it has usually never really 'gone away' completely.

A cancer may come back in one of several ways:

• the cancer may reappear at or near its original site. For example, a bowel cancer may be removed surgically but then, months or years later, the tumour may come back at the edge of the previous operation. This is known as a local recurrence.
• the cancer may reappear as secondary cancers in other parts of the body. For example, a breast cancer can be removed surgically but sometime later seedlings from that cancer might appear as secondary cancers in the bones. These secondary cancers are also known as metastases.

When a local recurrence appears, some time after previous treatment, this is because there were microscopic threads of cancer cells, spreading out into the surrounding tissues, which were left behind. Because these threads were so small they would have been invisible to the naked eye and so would not have been seen by the surgeon during the operation, nor would they have shown up on any scans or other tests done at the time. Although treatments for primary cancers always include a margin of normal healthy tissue around the cancer, to minimise the risk of a local recurrence, every so often there will still be tiny clusters of cancer cells left behind which, in the fullness of time, will grow again and so cause the cancer to 'come back'.

If a primary cancer has been removed completely then it cannot spread. But sometimes a cancer will have sent off seedlings of tumour into the blood stream or lymphatic system before it was removed. These will travel to other parts of the body and form metastases, or secondary cancers. To begin with these secondaries will contain only small numbers of cancers cells and will be completely invisible, undetectable and cause no symptoms. So tests done at this time will show no evidence of any secondary cancer. But, months or years later, those undetectable seedlings, which had spread before the primary cancer was treated, will grow to the stage where they cause problems and so the cancer will have 'come back'.

In a similar way, more advanced cancers which have spread to different parts of the body can 'come back' after apparently successful treatment with chemotherapy or hormones. The treatment may cause all the symptoms and all other evidence of the cancer to disappear, so that it no longer shows up on tests and the patient feels quite well. This is often called a remission. But although every trace of the cancer will have gone (a complete remission) often there will still be tiny remnants of clusters of cancer cells which have not been killed off by the treatment. These invisible, undetectable 'resistant' cells will start to multiply and eventually will form new secondary cancers, causing a relapse, with the cancer 'coming back'.

Very occasionally a cancer can appear to 'come back' because an entirely new tumour has developed near to where the first cancer was. This can happen but is much less common than a local recurrence or secondary spread from the original cancer. .