How does cancer start, grow and spread?

Cancer starts with changes in one cell or a small group of cells in the body. The changes make cells start to grow and multiply too much.

Body tissues grow by increasing the number of cells. Cells in many tissues in the body divide and grow very quickly until we become adults.

Unlike normal cells, cancer cells don't stop growing and dividing when there are enough of them. So the cells keep doubling, forming a lump (tumour) that grows in size.

Cancers can be grouped according to the type of cell they start from. There are 5 main types. The most common type is carcinoma.

To start with, cancer cells stay inside the body tissue from which they have developed. The cancer cells grow and divide to create more cells and will eventually form a tumour. Once a cancer can stimulate blood vessel growth, it can grow bigger.

The place where a cancer starts in the body is called the primary cancer or primary site. Cells from the primary site may break away and spread to other parts of the body.

Cells can break away from a cancer and spread in the blood or lymphatic systems to almost anywhere in the body. But most types of cancer tend to spread to one or 2 places in the body.

Hearing that cancer may come back can be frightening. Cancer might come back because the original treatment didn't get rid of all the cancer cells. Or because some cancer cells have spread elsewhere in the body and started growing there.