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 might come back some time after the first treatment. This idea can be frightening. There are different reasons for why cancer might come back. These reasons are:

  • the original treatment didn't get rid of all the cancer cells and those left behind grew into a new tumour
  • some cancer cells have spread elsewhere in 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. These are called 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. This is sometimes called adjuvant treatment.

The extra treatment might be on ore more of the folllowing:

  • chemotherapy
  • radiotherapy
  • hormone therapy
  • a targeted cancer drug Open a glossary item

These treatments aim to try to control or kill any cancer cells left.

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 didn't destroy all the cancer cells. 

Chemotherapy drugs kill cancer cells. They do this 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.

It is unlikely that any chemotherapy treatment kills every single cancer cell in the body. Doctors try to reduce the number of cancer cells as much as possible. The immune system kills off the remaining cells or they may die off. 

Find out 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.

Read more about radiotherapy treatment

Immunotherapy and targeted cancer drugs
Immunotherapy uses our immune system Open a glossary item to fight cancer. Targeted cancer drugs work by ‘targeting’ the differences in cancer cells that help them to grow and survive. Some drugs work in more than one way and may be both a targeted and immunotherapy treatment 

Some immunotherapies or targeted cancer drugs 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.

Read more about:


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. You can sometimes have a different type of treatment if this happens. But sometimes cancers develop resistance to many drugs at the same time. This is called 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'. This is so 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. This might include hormone therapy or a targeted cancer drug. This is called 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 might 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 might say that you are cured.

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. This is especially so 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. 

Find information about counselling organisations

You can phone the Cancer Research UK nurses if you would like to talk to someone outside your own friends and family. Talk to the Cancer Research UK nurses on freephone 0808 800 4040, from 9am to 5pm, Monday to Friday. 

Read about mental health and 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:

Next review due: 9 October 2026

Last reviewed

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