Why some cancers come back

This page tells you about how cancer can sometimes come back after treatment. There is information about

Why cancer might come back

Cancer may come back some time after its initial treatment. This idea can be frightening. There are different reasons for why cancer might come back. 

One reason is that the original treatment did not get rid of all the cancer cells and those left behind grew into a new tumour. Another is that some cancer cells have spread to other parts of the body and started growing there to form a tumour.

After surgery

Cancer can come back after surgery because:

  • some cancer cells were left behind during the operation
  • some cancer cells had already broken away from the primary cancer but were too small to see (micrometastases Open a glossary item)

Surgeons do their best to remove all of the cancer during surgery. But it is always possible to leave behind a small group of cancer cells. Your surgeon may recommend more treatment if they feel that there is a risk that the cancer could come back.

The extra treatment might be chemotherapy, radiotherapy, hormone therapy or a targeted cancer drug. These treatments aim to try to control or kill any cancer cells left.

You can read about surgery for cancer.  

After cancer drug treatment or radiotherapy

Cancer may sometimes come back after cancer drug treatment or radiotherapy. This can happen because the treatment did not destroy all the cancer cells. 

Chemotherapy drugs kill cancer cells by attacking cells that are in the process of doubling to form 2 new cells. But not all the cells in a cancer divide at the same time. Normal cells go into a long rest period between divisions. Cancer cells do too, although the rest period may be much shorter. 

Giving chemotherapy in a series of treatments helps to catch as many cells dividing as possible. Cells that were resting when you had your first treatment, may be dividing when you have your next and so will be more likely to die.

But it is unlikely that any chemotherapy treatment will kill every single cancer cell in the body. Doctors try to reduce the number of cancer cells as much as possible. The remaining cells will be killed off by the body's own defences (immune system) or may die off naturally.

You might find it helpful to read more about how chemotherapy works

Radiotherapy makes small breaks in the DNA inside the cells. These breaks stop cancer cells from growing and dividing and often make them die. Normal cells close to the cancer can also become damaged by radiation, but most recover and go back to working normally. If radiotherapy doesn't kill all of the cancer cells, they will regrow at some point in the future.

We have more information about radiotherapy treatment

Targeted cancer drugs work by 'targeting' those differences that help a cancer cell to survive and grow. Some treatments may get rid of a cancer completely. Others may shrink the cancer or control it for some months or years. So a cancer may seem to have gone and may not show up on any scans or blood tests. But there may be a small group of cells that remain in the body. They can start to grow again after a while or when the treatment stops.

You can read more about how targeted cancer drugs work

Cancers can become resistant to treatment

Sometimes cancer can become resistant to cancer drug treatment. Cancers develop from normal cells that have changed or mutated to become cancerous. The mutation Open a glossary item happens in the genes of the cell. These gene changes make the cell behave differently to a normal cell. Cancer cells can continue to mutate so that they become more and more abnormal.

Some mutations can make the cells resistant to cancer drugs such as chemotherapy, targeted cancer drugs or hormone therapy. If this happens, you can sometimes have a different type of treatment. But unfortunately, sometimes cancers develop resistance to many drugs at the same time. Doctors call this multi drug resistance. 

Scientists have found a group of genetic mutations that they think can cause drug resistance. These mutations mean that the cancer cell can keep the drugs out. The resistant cells have high levels of a substance called p-glycoprotein. P-glycoprotein is a protein found in cell walls. The protein acts as a pump and removes toxins from cells. Cells with high p-glycoprotein levels are very good at keeping cancer drugs out.

Researchers have been looking at drug resistance for almost as long as they have used cancer drugs. To make cancer drug treatment more effective, we need to find a way of overcoming resistance.

Cure or remission

These days, doctors are able to cure many cancers. But some cancers can come back many years after treatment. So you may find that your doctor is very unwilling to use the word 'cure', even though there is no sign that you have any cancer left. Doctors usually say that your cancer is in remission. This means that there is no sign of cancer in your body. If there are any cancer cells left:

  • there are too few to find
  • there are too few to cause any symptoms
  • they are in an inactive state and are not growing

Doctors can't be sure that the cancer has completely gone after treatment. So they may suggest that you have some type of long term treatment, such as hormone therapy or a targeted cancer drug. Your doctor may call this adjuvant treatment. 

Adjuvant treatment can also be a course of chemotherapy or radiotherapy after surgery. The aim of this treatment is to try to prevent cancer from coming back.

Living with uncertainty

It can be very difficult to live with the fact that your cancer may come back. Even if doctors tell you that they are 95% certain your cancer has gone for good, you may find it very upsetting that no one can say for sure that you're cured.

Some people find that they can't stop thinking about it even after the end of their treatment. You may feel a little frightened of planning anything in the future or you may feel sad or depressed.

For most people who are in this situation, each day lowers the risk of a recurrence. Most cancers that are going to come back will do so in the first 2 years or so after treatment. After 5 years, you are even less likely to get a recurrence. For some types of cancer, after 10 years your doctor may say that you are cured.

Unfortunately, some types of cancer can come back many years after they were first diagnosed. Some people find it very difficult to cope with this idea, but there are some things you can do to help. 

Getting help and support

You may find it helpful to talk to other people in the same situation if you are finding it hard to cope with the fact that you have had cancer. Or you could talk to a trained counsellor. This can help you to find ways of dealing with the fear and worry. 

You can get in touch with a counsellor by contacting one of the counselling organisations.

You can also look at our section about coping emotionally with cancer.

You can share your experiences with other people and find out how they coped by using our online forum, Cancer Chat.

Related information

You may find it helpful to read our information about how cancers grow.

We also have information about how cancer can spread.

You can read about cancer treatments.

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