Cancer is a collection of diseases that have in common uncontrolled cell growth and the ability to invade the body. Cancer can spread locally (in the region where it arose) and travel to sites of the body distant from where it arose. Distant spread is called metastasis. The ability to spread makes cancer a malignant disease. Malignant means that the disease, if not treated, can progress and lead to death.

To begin to understand the complexities of why cancer appears, it is necessary to know something about cell growth. The body contains several types of tissue such as muscle, bone, bone marrow (where blood is made) and blood. Tissues are composed of cells. Normal cells respond to specific chemical messages from other cells that either stimulate or inhibit their growth. Normal cells have a life span; they originate from primitive (young) cells called stem cells, reproduce (proliferate/divide), mature at varying rates and die. Mature cells cannot reproduce. Cells that die are generally replaced by new cells; this normal process is called renewal.

Most cancers originate in one or more young cells that have lost their built-in instructions to mature. Cancer cells reproduce repeatedly, replicating (copying) the abnormalities that have occurred. These cells are said to be immortal because, in contrast to normal cells, they appear to have an unlimited ability to reproduce. These altered cells do not respond to the normal chemical messages sent to them; they grow, even when their surroundings contain none of the signals normally required to stimulate growth. Their growth is out of control. As a result, they tend to reproduce at a rate which exceeds the rate needed to replace dying cells. They can form tumours (clusters of abnormally growing cells) and/or invade tissues, including bone marrow and blood.

Cancers are classified by both the tissue of origin and the cell type. For example, osteo in osteosarcoma means the cancer arose in bone. Sarcoma is a term used for tumour cells that resemble embryonic connective tissue cells. In general, a tumour that does not invade is considered benign (not cancer, not malignant). Some tumours are of borderline malignancy; under some conditions they will remain in a local area, but under other conditions - often poorly understood - they may acquire the capacity to invade and spread to other areas of the body. Some tumour cells continue to modify their properties as they grow.

© Public Health Agency of Canada, 2004