Introduction

A cancer is a group of cells (usually derived from a single cell) that has lost its normal control mechanisms and thus has unregulated growth. Cancerous (malignant) cells can develop from any tissue within any organ. As cancerous cells grow and multiply, they form a mass of cancerous tissue--called a tumor--that invades and destroys normal adjacent tissues. The term "tumor" refers to an abnormal growth or mass; tumors can be cancerous or noncancerous. Cancerous cells from the primary (initial) site can spread (metastasize) throughout the body.

See the sidebar Talking About Cancer.