What is cancer?

Cancer starts in cells. Cells are tiny building blocks that make up the body's organs and tissues. Usually, these cells divide to make new cells in a controlled way. This is how our bodies grow, heal and repair.

Sometimes, this goes wrong and the cell becomes abnormal. The abnormal cell keeps dividing and making more and more abnormal cells. These cells form a lump, which is called a tumour.

Not all lumps are cancerous. A lump that is not cancerous (benign) cannot spread to anywhere else in the body. A lump that is cancer (malignant) can grow into surrounding tissue.

Cancer cells sometimes break away from the primary cancer and travel through the blood or lymphatic system to other parts of the body. Cancer cells that spread and develop into a tumour somewhere else in the body are called a secondary cancer.

It’s important for doctors to know what type of cancer a person has. The type of cancer is generally based on the part of your body and the type of cell where the cancer first developed.

The most common places for cancer to develop are the skin, lungs, breasts, prostate, colon, and rectum.

There are 3 main types of cell where cancer develops are:

  • epithelial cells – cancers that develop in this type of cell are called carcinomas; about 80 to 90% of cancers are this type
  • cells of the blood and lymphatic system – cancers that develop in this type of cell are called leukaemias and lymphomas; about 7% of cancers are this type
  • connective tissue cells – cancers that develop in this type of cell are called sarcomas; about 1% of cancers are this type

Cancer can sometimes develop in other types of cell, but this is rare. Brain tumours are the most common cancers in this group.

Last updated:
18 April 2022

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