While cancer starts in just one cell, too small to be seen with the naked eye, it grows into a tumor big enough to be seen over months to many years. For instance, a breast tumor may be enlarging for 10 years before it even be- comes visible on a mammogram! By the time a tumor is visible on an X-ray, it has already gone through most of it's divisions and contains at least 1 billion cells. These cells can spread at least 4 different ways: 1) The cancer can be "locally invasive", that is grow by extending into other nearby tissues and organs. A group of cells makes up a tissue, and a group of tissues forms an organ. A tumor starting in the brain can press on vital structures and erode into crucial blood vessels, and it is critical to get "local control" of a cancer in order to cure it. 2) Cancer can spread through the bloodstream, by individual cancer cells breaking off of the main tumor and being carried away to distant sites in the body, where they can start to divide. This process is called metastasis. While cancer can metastasize to any area of the body, some sites are more hospitable; so cancer preferentially spreads to these sites. The most common areas for metastasis through the bloodstream, called "hematogenous metastasis" by doctors, are the liver, lung, bone and brain. 3) The disease may spread through the "lymph system", that is an elaborate array of bean-sized filters, called "lymph nodes" that purify the blood. These lymph nodes contain white blood cells to kill germs, and often swell in response to infection. For example, "swollen glands" in the neck with a sore throat result from lymph nodes in the neck enlarging to fight a throat infection. Thus they are crucial for the immune system. Lymph nodes form groups, and the groups are connected together by lymph channels. Eventually the purified blood serum is returned to the regular bloodstream. Nearby lymph nodes are often the first site of cancer spread. This is then called "lymphogenous metastasis". Therefore local lymph nodes are usually checked to gauge how extensive the cancer is. Some cancers, like Hodgkin's Disease, even start from the normal cells in lymph nodes. 4) Cancers in specific locations may have other parti- cular ways of spreading. As examples, tumors in the brain may spread through the spinal fluid, which bathes and cushions the brain, while abdominal tumors may spread via the membranous lining of the abdominal cavity, called "intraperitoneal" spread. copyright©CancerAnswers, 1994 all rights reserved last updated April 10, 2011