What is Cancer?

Cancer starts when the cells of an organ or tissue in the body become abnormal. They grow and multiply out of control. Normal cells have a life cycle. They reproduce themselves throughout the body to replace worn out tissue, to heal wounds and to maintain healthy organs. When something happens and cells grow out of control they usually form a mass, called a tumour.

Some tumours grow only at the site where they begin (locally). These are called benign tumours. Other tumours grow locally but they might also invade and destroy the normal tissue around them or they might spread to distant parts of the body. These tumours are called malignant tumours or cancers.

Sometimes malignant cells break loose from the original (primary) tumour, get carried to other parts of the body and start growing in the new site as an independent secondary cancer. A tumour that has spread in this manner has ‘metastasized’ and the secondary tumour (or tumours) is called a metastasis (or metastases).

What causes cancer?
Most cancers occur by chance (randomly) in people over the age of 50 as the result of damage to their genes. Genes play a role in all cancers. Many genes in the human body help to control how the cells divide and grow. When changes (called mutations) occur in those genes, they may lose that control over the cells. Because most cancers do not happen until a cell is affected by several gene mutations, most cancers are not seen until later in life. Gene mutations may be caused by aging, exposure to chemicals, radiation, hormones or other factors within the body and the environment. Over time, a number of gene mutations may occur in a cell, allowing it to divide and grow in a way that becomes a cancer.

Metastasis: How Cancer Spreads
Tumours are called malignant because they have the ability to invade normal tissues (replacing healthy cells with cancer cells) and to metastasize (spread) to other parts of the body. Often, cancer deaths don’t come from the primary site where the cancer first began, but from the metastases. For example, a patient with stomach cancer may actually die from liver failure after the cancer has spread to the liver.

When one type of cancer spreads to another part of the body, it doesn’t become another type of cancer. For example, if a person with colon cancer develops a metastasis in the lung, then the tumour growing in the lung has the same features as the colon cancer. It is the same cancer in a new place. This is why it is very important for the doctors treating a patient to be able to find the primary site where the cancer started.

Metastasis takes place in many ways: through the lymphatic system, through the blood, by spreading through body spaces such as the abdominal cavity, or through implantation. Cancers can spread by more than one route.

Lymphatic System
The most common way for cancer to spread is through the lymphatic system. The lymph system has its own channels that circulate throughout the body, similar to the veins and arteries of the bloodstream. These channels are very small and carry fluid called lymph throughout the body.

Often when a solid tumour is removed by surgery, the surgeon will remove not only the tumour but the neighboring lymph glands, even though there is no visible sign of cancer in those glands. This is done as a precautionary measure, because if even one cell has broken away from the tumour and lodged in the lymphatic system, the cancer could continue growing and metastasizing.

Circulatory System
Cancer can metastasize through the blood. All cells (healthy and cancer) must have a blood supply in order to live, so all cancer cells have access to the bloodstream. Malignant cells can break off from the tumour and travel through the bloodstream until they find a suitable place to start growing a new tumour. Tumours that spread by blood almost always metastasize through the veins rather than through the arteries. Sarcomas spread through the bloodstream, as do certain types of carcinomas, like carcinoma of the kidneys, testicular carcinoma, and Wilms' tumour, a type of kidney cancer seen in young children.

Local Invasion
Cancers can spread by local invasion -- by intruding on the healthy tissue that surrounds the tumour. Some cancers that spread this way do not travel very far from the original site. An example of this kind of cancer is basal cell carcinoma of the skin. When this kind of cancer is removed by surgeon, a wide area of healthy tissue surrounding it is also removed and it is usually "cured" immediately. Unless some cells have been left behind, it is very unlikely that it will recur. However, it is possible that a second cancer of the same kind may start to grow at a later time at a completely different site -- the new growth would not be connected with the first.

Implantation
A very rare type of metastasis is caused by implantation or inoculation. This can happen when a biopsy is done or when cancer surgery is performed. Malignant cells may accidentally drip from a needle or an instrument (this is also called a "spill"). That is why, if the cancer is small enough, the surgeon tries to avoid spills by removing it completely at the initial surgery (at the time of the biopsy).

Cancers do not spread in a completely random fashion. Some parts of the body accept metastases more easily than others. For example, cancers rarely metastasize to the skin, but they often metastasize to the liver and lungs. Each type of cancer has its own pattern for metastases. See the individual cancer pages for more information.

Can cancer be prevented?
Over half of all cancers can be prevented. Don’t smoke, or quit smoking.  Exercise and eat a healthy diet.

Hereditary Cancer
Most cancers are not caused by an inherited (hereditary) risk. For information about hereditary or inherited risk, see our Hereditary Cancer Program information.