The digestive system processes food for energy and rids the body of solid waste. After food is chewed and swallowed, it enters the esophagus, a tube that carries food through the neck and chest to the stomach. The esophagus joins the stomach just beneath the diaphragm (the breathing muscle under the lungs).

The stomach is a sac-like organ that holds food and begins the digestive process by secreting gastric juice. The food and gastric juices are mixed into a thick fluid called chyme that is then emptied into the small intestine. The small intestine continues breaking down the food and absorbs most of the nutrients. It is the longest section of the gastrointestinal (GI) tract, measuring over 20 feet.

The small intestine joins the colon, a muscular tube of about 5 feet long. The colon absorbs water and mineral nutrients from the food matter and serves as a storage place for waste. The waste left after this process goes into the rectum. From there it passes out of the body through the anus.

Gastrointestinal Stromal Tumors

Gastrointestinal stromal tumors (GISTs) are fairly rare tumors of the GI tract. In the past, some were thought to start in the muscle layer of the GI tract, and some were thought to start in nerve cells. This is because the tumor cells look like muscle or nerve cells under the microscope. They were often thought to be related to sarcomas -- cancers that start in the muscle, bones of the legs and arms, or other connective tissues.

We now know that these tumors are in fact not true muscle or nerve tumors. Most doctors believe they start in special cells found in the wall of the GI tract, called the interstitial cells of Cajal (ICCs), or in very early cells that can develop into ICCs.

ICCs are part of the autonomic nervous system, which sends signals to the GI tract. Some have called these cells the "pacemakers" of the GI tract. The nerve signals they send cause muscles of the digestive organs to contract, which helps to move food and liquid through the GI tract.

Not all GISTs are cancerous. Some are benign -- they don't invade into other areas or spread to other parts of the body. Ways to try to determine whether a GIST is benign or cancerous are discussed further in the section "How Are Gastrointestinal Stromal Tumors Diagnosed?"

Difference Between GISTs and Other GI Tract Tumors

Cancers can occur anywhere in the GI tract from the esophagus to the anus.

Most cancers in the GI tract start in glandular cells lining the GI tract. These cells can develop into adenomas (non-cancerous tumors of gland cells) or adenocarcinomas (cancers of gland cells).

In some parts of the GI tract, mostly in the upper part of the esophagus and at the end of the anus, there are flat cells called squamous cells. These are the same type of cells that are found on the surface of the skin. Cancers starting in these cells are called squamous cell carcinomas.

Neuroendocrine cancers such as carcinoid tumors are a relatively uncommon type of cancer that can also occur in the gastrointestinal tract. They develop from neuroendocrine cells, which have some features of nerve cells and in other ways resemble hormone producing (endocrine) cells.

GISTs are different from these more common GI tract cancers, in large part because they start in different types of cells. GISTs are also quite different in their prognosis (outlook for survival) and their treatment. For these reasons, doctors need to figure out whether a patient has a GIST, an adenoma, an adenocarcinoma, a neuroendocrine cancer, some other type of tumor, or a non-cancerous condition.

It is important for patients to understand that GISTs are not the same as other, more common types of GI tumors. By learning more about GISTs, patients can better take part in their health care and make informed decisions about treatment options.