CNS A cancer is a group of abnormal cells, known as a tumor, that grows uncontrollably.
Cancerous tumors invade and destroy surrounding tissue.
Not all tumors are cancerous. Benign tumors, which are not cancerous, do not invade and destroy tissue. However, a benign tumor may grow very large. Cancerous tumors may shed cancer cells that spread to other parts of the body, which is called a metastasis . Benign tumors do not spread in this way.