Cancer happens when cells somewhere in the body lose the ability to control their growth rate and start to divide uncontrollably. Cancer is caused by genetic changes called mutations that make cells less able to respond to the signals that normally keep them from growing and dividing too fast. The disease is most common in tissues where the cells need to divide a lot normally because there is a higher chance of mutations in these cells. The type of cancer that occurs depends on where in the body those mutations first occur.

Cancer terminology can be quite confusing, so here are some definitions to help you understand what the different terms actually mean:

• Tumor — an abnormal growth made up of a mass of dividing cells. Tumors can either be benign or malignant.
• Benign — a term used to describe a tumor that has not invaded neighboring tissues within the same organ or region. This does not mean that benign tumors are not large, only that the cells that make them up do not spill over into territory that is normally occupied by other types of cells. You can also think of benign as meaning “not cancerous.”
• Malignant — a term used to describe a tumor that has invaded tissues surrounding the cells in which it started and that might be capable of spreading to other parts of the body (metastasis). You can also think of malignant as meaning “cancerous.”
• Cancer — a malignant tumor arising from cells dividing uncontrollably that then invade surrounding tissues. If a tumor does not invade tissues beyond where it started, it is not cancer but instead is a benign tumor.
• Metastasis — means cancer has spread beyond the site where it originated to other organs. If a cancer has not yet metastasized, it is much easier to treat. Without treatment, all cancers will eventually metastasize throughout the body; this is why early detection of cancer is so critical.