WASHINGTON (CNN) -- A leading organization of cancer doctors announced Wednesday that only two screening methods were considered reliable when checking patients for a recurrence of colon cancer.

The American Society of Clinical Oncology (ASCO), which represents 12,000 cancer doctors, reviewed available research on several screening methods and recommended only an annual physical exam and a colonoscopy screening every three to five years.

It said colonoscopy could be performed much less often. Currently, colonoscopies commonly done once or twice a year in people who have had colon cancer.

In a colonoscopy, a camera is put on the end of a tube, which is then threaded up into the colon to look for the small polyps that can grow into tumors and spread.

Other scanning methods reviewed included CT scans, chest X-rays, blood counts and tests of liver function. The review found that such tests are either not sensitive enough, or there is no data showing that they work.

Colon cancer third most common cancer killer in U.S.

It is estimated that some 56,000 Americans will die of colon cancer this year; it is one of the most deadly cancers.

ASCO noted that lifestyle changes are believed to reduce the risk of developing colon cancer and that a diet low in fat and high in fruits, vegetables and whole grains is important for prevention.

The ASCO also commented on two promising tests for specific sub-groups of patients. It said the CEA test might be worth doing in some cases. The test looks for a protein that some tumors produce, however, about 30 percent of colon cancers do not produce the protein.

It might also be useful to do an exam called a proctosigmoidoscopy in some patients who were not treated with radiation," the ASCO said. The method is similar to a partial colonoscopy.

Earlier this month, a report in the Journal of the National Cancer Institute said a fecal occult blood test, which looks for traces of blood in the stool, reduces the risk of colon cancer by 21 percent when done every other year.