Staging is the process of finding out how far a cancer has spread from its original site. For many cancers, its stage can be the most important factor in selecting treatment options. It is also often helpful in predicting the patient's prognosis (the course of the disease and outlook) .To help stage the tumor, doctors may use several types of imaging, endoscopy, or other tests, such as those described in the previous section ("How are gastrointestinal stromal tumors diagnosed?").

Doctors describe the extent of spread of most types of cancer by using a staging system. The most common system used is the TNM system of the American Joint Committee on Cancer (AJCC). (T stands for the size of the tumor, N stands for spread to lymph nodes, and M is for distant metastasis.) At this time there is no specific TNM staging system for GISTs. Some doctors use the TNM system for soft tissue sarcomas to stage these tumors, but it is not yet clear how well this applies. (For more information on this staging system, see Sarcoma -- Adult Soft Tissue Cancer.)

Because there is no formal staging system for GIST, doctors often use other factors to select the best treatment and to assess the outlook for patients.

Prognostic factors

For GISTs, factors such as the size of the tumor, its grade, and whether it has already shown signs of spread in the abdomen can be helpful in predicting how fast the cancer will grow and how likely it is to come back after treatment.

The size of the tumor can give some indication of prognosis (outlook). Smaller tumors (generally those less than 2 inches across) tend to be less likely to spread and have better outcomes than larger ones. The best outlook is for tumors smaller than 1 inch.

Another factor in prognosis is the tumor's histologic grade - its tendency to grow and spread more rapidly based on how the cells look under the microscope. For GISTs, this is done by finding out how many dividing cells, or mitoses, there are. The pathologist looks at many different areas (or fields) of a tumor sample under the microscope and counts the actual number of dividing cells. By doing these counts, the doctor comes up with a number called mitoses per high-powered field. This number will help predict whether the GIST is most likely to be a cancer or a benign tumor.

Doctors often use these factors to separate patients into very low-, low-, intermediate-, and high-risk groups. These groupings describe the risk of the cancer spreading to distant areas in the body.

Whether or not a tumor has already spread to other areas when it is first found is an important factor in determining outlook. It may have an effect on whether certain treatment options, such as surgery, are available.

When these tumors are cancerous and spread away from the initial (primary) site, they most often spread within the abdomen, and, in particular, to the liver. Less often, they may also spread to the lungs and bone. Spread to the lymph nodes, brain, and elsewhere is not common. About half the time, the tumor is still localized when it is found; one fourth of the time it has spread to nearby tissues and one fourth of the time it has spread to distant sites.

Survival rates

It is very hard to get accurate numbers on survival rates for GISTs. Part of this is because these tumors are fairly rare to begin with. In the past, they were often classified as other types of cancers, which made the numbers available for study even smaller. Finally, treatment has changed dramatically in the past few years now that newer, targeted therapies are being used. The survival rates now available are based on people treated 5 or more years ago, before these treatments were around, so people being treated for GISTs today are likely to have a better outlook.

Based on people treated between 1992 and 2000, the overall relative 5-year survival rate of people diagnosed with a malignant GIST was estimated to be about 45%. If the tumor was confined to the organ where it started, the 5-year relative survival was 64%. If it had grown into nearby tissue, the 5-year relative survival was around 30%. If it had spread to distant sites when it was first diagnosed, the 5-year relative survival was 13%. Again, the numbers for people now being diagnosed with GISTs are likely to be much better.

The 5-year survival rate refers to the percentage of patients who live at least 5 years after their cancer is diagnosed. Five-year rates are used to produce a standard way of discussing prognosis. Of course, many people live much longer than 5 years. Five-year relative survival rates assume that people will die of other causes and compare the observed survival of people with GISTs with that expected for people without GISTs. That means that relative survival only includes deaths from GISTs.

Last Medical Review: 05/11/2009
Last Revised: 05/11/2009