Metastasis is the spread of cancer cells from the original site to other parts of the body.

What is going on in the body?

As cancer grows and changes, cells lose their ability to stay in a single tumor mass. When cancer cells get into the lymph system or blood vessels, they break off. Cells can spread as a single cell or as many tiny cells. The cells circulate until they find a good place to grow. The entire body is supplied by blood and drained by the lymph system.

Metastases can appear in almost any part of the body. A site for metastatic cancer must have a blood supply and an environment where it can grow. Cancer cells may travel throughout the body and take hold in certain locations. Common sites are the brain, lungs, liver, lymph nodes, bone, skin, and adrenal glands.

Some cancers already involve much of the body when they are first detected. This is common with cancers such as leukemia and lymphoma. Where and when a cancer will spread depends on many factors. These factors include the primary tissue type and the genetic abnormality of the cancer.

What are the causes and risks of the disease?

The particular genetic changes in the cancer may make it more prone to spread. Metastasis in part depends on how different the cancer is from surrounding tissue, whether certain genes are activated, and how close the cancer is to lymph nodes or blood vessels. Other factors are the strength of the person's immune system and how long a malignancy has gone untreated. Anyone with a malignancy is at risk for metastases. There are a few cancers that rarely spread. These include primary brain tumors and basal cell or squamous cell carcinoma of the skin.