About ninety percent of the deaths due to cancer

involve tumors that have spread around the body. The movement of tumor cells to

other parts of the body

is known as metastasis. Metastasis is a complex process

During which cancer cells break off the original

or primary tumor and move through the body to form tumors at new locations

From the point of view of a cancer cell, this is a dangerous

and often unsuccessful process A trip through the body is full of hazards that

cause the death of most cells that begin the journey

even tough cancer cells. To begin the process

individual cells must break away from the tumor and invade nearby vessels

The cells crawl along the surface of other cells

and the fibrous stringy structures surrounding them

and then force their way in. Shown here

is the invasion of the blood supply. Once inside a blood vessel,

the cancer cells may parish from a variety of causes

Some cells die simply because they are unable to survive

floating around in the bloodstream.

Others may become damaged and die when they squeeze through tight spaces

or bump into the walls of the blood

vessels.

Still other migrating cells may be recognized and destroyed

by cells of the immune system.

How and where the migrating cells stop is different for different cancer types

Once the tumor cells are no longer moving

they can begin the process of forming a new tumor

by leaving the blood vessel and beginning to reproduce in the new

location

This does not always occur and cells that have made it this far may still die

or fail to divide If the new environment

is suitable the newly-arrived cell will begin to grow

and a new tumor will develop.