Tumors can be benign (non-cancerous) or malignant (cancerous).

Benign tumors remain contained in one site and do not spread to other sites. Some benign tumors do continue to grow and may need to be removed. Others do not grow above a certain size or cause any symptoms and, thus, never need to be removed.

Malignant tumors have an unlimited potential to grow and to spread from the organ of the body where they started (eg, breast, prostate, bowel) to other parts of the body. These malignant tumors are referred to as cancer. The spread of cancer from one part of the body to another is called metastasis.

The organ where a cancer originated is called the "primary cancer site" and this site gives cancer its name (eg, breast cancer, prostate cancer, bowel cancer).

Cancer that has spread from the primary site to another organ (eg, the liver or lungs) is called metastatic or secondary cancer.

NB: Some cancers do not form tumors. For example, leukemia is a cancer of the bone marrow and blood.