How does cancer spread?

A malignant tumor is made up of cancer cells. When it first develops, this malignant tumor may be confined to its original site. This is known as a cancer in situ (or carcinoma in situ). If these cells are not treated, they may spread beyond their normal boundaries and into surrounding tissues, becoming invasive cancer. Some benign tumors are precancerous and may progress to cancer if left untreated. Other benign tumors do not develop into cancer.

For a cancer to grow bigger than the head of a pin, it must grow its own blood vessels. This is called angiogenesis. Sometimes cells move away from the original (primary) cancer, either by the local tissue fluid channels (lymphatics) or in the blood stream, and invade other organs. When these cells reach a new site, they may continue to grow and form another tumor at that site. This is called a secondary cancer or metastasis.