What is cancer?
Normal cells have a life cycle. They divide and grow throughout the body to replace worn out tissue, heal wounds and maintain healthy organs.
Cancer starts when cells in the body become abnormal. They start growing and multiplying out of control. When they do this, they sometimes form a mass called a tumour. Cancer cells make it hard for your body to work properly.
Some cancers grow only in the place where they started (localized). Other cancers may grow and invade the normal tissue around them (local invasion).
Another name for cancer cells is malignant. Malignant cells can grow into other tissues and spread around the body.
Sometimes tumours are made out of cells that are not cancerous (not malignant). They cannot invade the tissue around them or spread around the body. These are called benign tumours.
Watch the Canadian Cancer Society's video What is Cancer?
Cancer cells can sometimes break loose from the original (primary) tumour. Metastasis is when these cells get carried to other parts of the body and start growing. The tumour in the new site is called a secondary tumour.
Cancers do not spread in a completely random way. Metastasis mainly happens through the lymphatic system or the circulatory system.
Lymphatic System: The most common way for cancer to spread is through the lymphatic system, which carries lymph fluid throughout the body. Often, a surgeon will remove the tumour and the lymph nodes nearby, even if there is no sign of cancer in the lymph nodes. This is done in case even one cancer cell has broken away from the tumour and travelled into the lymph node.
Circulatory System: Your circulatory system carries blood throughout your body. Cancer cells can break off from the tumour and travel through the bloodstream. Tumours that spread by the blood almost always do so through the veins, not the arteries.
Some parts of the body accept cancer cells more easily than others. For example, cancers rarely metastasize to the skin. But, cancers often spread to the liver and lungs.
When one type of cancer spreads to another part of the body, it does not become another type of cancer. For example, if a person with colon cancer has a metastasis in the lung, the tumour growing in the lung is the same cancer as in the colon.
Many genes in the human body help control how cells divide and grow. When changes occur in your genes (called mutations), they may lose control over the cells. This can cause cancer.
Gene mutations can be caused by aging, chemicals, radiation, hormones, other things in the body, and the environment. Because most cancers happen by chance (randomly) and after a cell has had many gene mutations, most cancers are seen in people over the age of 50.
Most cancers are not hereditary (inherited) and caused by a gene mutation you got from a parent. For information about hereditary risk, see our Hereditary Cancer Program page.
There are lifestyle choices you can make that can lower your risk of getting cancer. For example, exercising more or eating a healthier diet can lower your cancer risk. For information on cancer prevention, please visit our Prevention site.
There is information about healthy living after cancer in our Life After Cancer.
The American Institute for Cancer Research has helpful recommendations to lower your risk of getting cancer.