Although tumors can start to grow in several places simultaneously, that is unusual.
They usually grow at a single site. So if cancer spreads, it is usually because small bits of cancer cells are cast off the original, or primary, tumor and travel to other parts of the body. There are three ways for the cancer to spread.
Direct Extension As the tumor mass grows, it invades the organs and tissues immediately next to it. It tends to form roots, growing into the layers of surrounding tissue like carrots growing into the earth.
Through the Blood (Hematogenous Spread) Tumors have a blood supply. Arteries pump blood into the malignant cells and veins take it away. Pieces of the tumor can grow through the walls of these blood vessels, enter the bloodstream and circulate around the body until they land in various organs. Fortunately, very few of these circulating cells actually find a place to grow.
Through the Lymphatic System There are two blood vessel systems in the body. One consists of arteries that move blood to all parts of the body, veins that return the blood to the heart, and small capillaries around the organs and tissues that allow oxygen to be transferred.
The second is the lymphatic system. This is a separate system of tiny blood vessels called lymphatics under the skin and throughout the body. These vessels carry a liquid called lymph. The purpose of lymph is to drain waste products such as infectious, foreign or otherwise toxic materials.
