Cancer comes in many shapes and sizes, and how it affects the body varies greatly. But we'll talk you through the basic definition of cancer and what causes itCancer is a term used to describe a group of illnesses all having certain common characteristics. These characteristics include an over-growth of cells that forms a tumor. There are over 200 different types of cancer, each with a specific name, treatment and chance of being cured.

What all cancers have in common is the lack of normal cell growth, which can result in serious health problems.

What does a tumor do?

Tumors cause medical problems in two ways:

  1. directly, by pressing on and damaging nearby organs,
  2. or indirectly, by breaking off and invading other distant tissues and organs.


To better understand what cancer is and how it occurs, we'll discuss how cells grow and reproduce normally.

1. Normal cell organisation

Cell multiplication is normal. We all start life as just one single cell, the ovum, which is fertilized by sperm and begins to multiply, producing more cells. These cells grow and mature to become nerve cells, muscle, blood cells or connective tissue (e.g., 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 manydifferent 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. Therefore, many different types of cells must work in co-operation with each other in order for an organ to work effectively.

2. Normal cell growth and repair

Occasionally, 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.

3. Tumors

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 cell grows so much it forms 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.

4. Benign and malignant tumors

Tumours can be either benign or malignant:

  • Benign tumours can cause problems in the organ where they occur, but do not spread. These tumours may be removed surgically or treated with drugs and/or radiation to reduce their size, which usually cures the disease.
  • Malignant tumours can spread—cells from the original tumour can sometimes spread from the original site. This spread happens in three ways:
    • By direct extension, where the cells literally 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.

Once the cancer spreads, the cells migrate to many other distant sites and organs in the body. Multiple growths may occur, affecting many parts of the body, eventually causing multiple organs to fail. These growths are known assecondary cancers or metastasis.

Organs or parts of the body that have a rich blood or lymph supply are the most likely places where malignant growths appear. These organs include the lungs, the liver, bone marrow and lymph nodes.


1. Carcinoma

Carcinomas are malignant tumours that arise from cells lining the surfaces of the body. For example, a cancer of the lining of the stomach is termed a carcinoma, as is cancer arising from cells lining the breast ducts. These are often just called cancers.

2. Sarcoma

Sarcomas are malignant tumours that arise out of cells in the supporting structures of the body (e.g. bone, muscle and cartilage).

3. Leukaemia, lymphoma and myeloma

Leukaemia, lymphoma and myeloma are all malignancies that arise fromblood cells or from cells that go to make up blood. These cancers are also known as hematological malignancies.