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What cancer is

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This page tells you what cancer is. There is information about


What cancer is

Cancer is a disease caused by normal cells changing so that they grow in an uncontrolled way. The uncontrolled growth causes a lump called a tumour to form. If not treated, the tumour can cause problems in one or more of the following ways

  • Spreading into normal tissues nearby
  • Causing pressure on other body structures
  • Spreading to other parts of the body through the lymphatic system or bloodstream

There are over 200 different types of cancer because there are over 200 different types of body cells. For example, cells that make up the lungs can cause a lung cancer. There are different cells in the lungs, so these may cause different types of lung cancer. 

Find out more about types of cells and cancer.


Normal cells

Your body is made up of billions of tiny cells that can only be seen under a microscope. These cells are grouped together to make up the tissues and organs of our bodies. They are a bit like building blocks.

Diagram showing how normal cells make up the tissue in our body

Different types of body tissues are made up of different types of body cells. For example, there are bone cells in bone and breast cells in the breast.


Genes and cancer

Different types of cells in the body do different jobs, but they are basically similar. They all have a centre called a nucleus. Inside the nucleus are the genes. Genes are really bits of code. The information they carry can be switched on or off. The genes control the cell. They decide when it will reproduce, what it does and even when it will die.

Diagram showing where genes are in cells

Normally the genes make sure that cells grow and reproduce in an orderly and controlled way. If the system goes wrong for any reason, the usual result is that the cell dies. Rarely, the system goes wrong in a way that allows a cell to keep on dividing until a lump called a 'tumour' is formed. 

You can look at information about the differences between normal cells and cancer cells.


Benign and malignant tumours

Tumours (lumps) can be benign or malignant. Benign means it is not cancer. Benign tumours

  • Usually grow quite slowly
  • Do not spread to other parts of the body
  • Usually have a covering made up of normal cells

Benign tumours are made up of cells that are quite similar to normal cells. They will only cause a problem if they

  • Grow very large
  • Become uncomfortable or unsightly
  • Press on other body organs
  • Take up space inside the skull (such as a brain tumour)
  • Release hormones that affect how the body works

Malignant tumours are made up of cancer cells. They

  • Usually grow faster than benign tumours
  • Spread into and destroy surrounding tissues
  • Spread to other parts of the body

The diagram below shows  a tumour appearing in cells lining a body structure such as the bowel wall. The tumour grows through the layer holding the cells in place (the basement membrane) and spreads into the bloodstream. 

Diagram showing a malignant tumour

It is the ability to spread that makes a cancer harmful. If a cancer is not treated, it can spread into the organs near to where it started growing. It can also damage other parts of the body that it spreads to.


Primary and secondary cancer

The place where a cancer begins is called the primary cancer. Cancers may also spread into nearby body tissues. For example, lung cancer can spread to the lining of the chest (the pleura). Ovarian cancer can spread to the lining of the abdomen (the peritoneum). This is called locally advanced cancer.

Cancer cells can break away from the primary tumour and be carried in the blood or lymphatic system to other parts of the body. There they can start to grow into new tumours. Tumours from cancers that have spread are called secondary cancers or metastases (pronounced met-as-tah-seez). A cancer that has spread has metastasised. The diagram shows a primary bowel cancer that has spread to the liver.

Diagram showing a primary and a secondary cancer

The various organs of the body are made up of different types of cells. Any of these cell types can grow into a primary cancer. Different types of cancer behave very differently. The type of cancer affects whether it is

  • Likely to grow quickly or slowly
  • Likely to produce chemicals that change the way the body works
  • Likely to spread in the blood or lymph system
  • Likely to respond well to particular treatments

Cancer symptoms

Cancers can cause different symptoms according to where they are in the body. A cancer may press on a nerve, or another nearby body organ. It may also cause symptoms by releasing chemicals or hormones into the bloodstream. 

You can find out about the symptoms for each type of cancer in our section about specific cancers.

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Updated: 24 July 2013