What is cancer?

Cancer is a disease of the cells in our body. All the cells in our body are constantly growing, dividing, dying and being replaced in a controlled way. Cancer occurs when normal cells grow and change in an abnormal and uncontrolled way.

When these cancer cells continue to grow, they can form a lump called a tumour or mass. Some cancers don't form lumps, such as cancers that affect the blood, like leukaemia.

Cancer is generally named after the organ or cell where the disease starts. For example, lung cancer begins in cells in the lung. This is known as the primary tumour or primary cancer.

Cancer sometimes spreads to other parts of the body. This is known as secondary cancer or metastasis.

You can watch our video about cancer to find out more

To better understand what cancer is and how it occurs, we need to learn more about how cells grow and reproduce normally.

Normal cell organisation

Cell multiplication is normal. We all start life as just one single cell, the ovum, which is fertilised by sperm and begins to multiply, producing more cells.

These cells grow and mature to become nerve cells, muscle cells, blood cells or connective tissue (for example, skin).

Groups of these different types of cells then come together to form our organs (eyes, ears, legs, lungs, skin, etc). Each of our organs is made up of many different types of cells and each cell has a particular function within that organ; the cells work in co-operation with each other to make the organ work effectively.

In the heart, for example, specific cells called cardiac muscle cells pump blood. However, nerve cells are also located in the heart. These cardiac nerve cells are needed to conduct the electrical signals to produce each contraction (what causes the heart to beat).

Therefore, many different types of cells must work in co-operation with each other in order for an organ to work effectively.

Normal cell growth and repair

Normally, cells within an organ die off or wear out and new cells then replace them. This growth of new cells is a highly complex and tightly-regulated process.

With cancer, for one reason or another, the growth of new cells becomes faulty. When a cancer cell begins to grow, rather than just replacing the cells that have been damaged or lost, it multiplies out of control, taking over the organ.

In some cases the cancer cells grow so much, they form a mass of cells called a tumour.

This tumour can cause health problems by blocking internal ducts, or by pressing against other organs, preventing them from working properly.

Benign and malignant tumours

Tumours can be either benign or malignant:

Benign tumours are not cancerous. They grow quite slowly. They can cause problems in the organ where they occur, but they do not spread. These tumours may be removed surgically or treated with drugs and/or radiation to reduce their size.

Malignant tumours are cancerous. They grow quicker than benign tumours and can spread to surrounding tissue or to other parts of the body.

This spread can happens in 3 ways:

• By direct extension, where the cells grow into nearby organs
• By the lymphatic system, where cells from the original tumour break off and enter the lymph canals
• By the blood stream

Carcinoma

These cancers begin in the epithelial cells in the skin or in the tissue that lines the internal organs. Subtypes of carcinoma include adenocarcinoma, basal cell carcinoma and squamous cell carcinoma.

Sarcoma

These cancers begin in the bone or soft tissue, such as fat, muscle, blood vessels and nerves.

Leukaemia

These cancers begins in the tissue that makes blood cells in the bone marrow. It affects white blood cells, which have a key role in the body's immune system, including fighting infection.

Lymphoma

These cancers begins in the lymphocytes, which are white blood cells that play a vital role in the immune system by fighting infection. With lymphoma, the lymphocytes build up in the lymphatic system.

Multiple myeloma

These cancers begins in the plasma cells, which are also found in the immune system. Plasma cells produce antibodies to fight off infection. 

Brain and spinal cord cancers

These cancers are known as central nervous system (CNS) cancers. There are different types of brain and spinal cord tumours based on the type of cell they affect and also where they begin within the CNS.