A new study questions the effectiveness of colonoscopy in reducing colorectal cancer (CRC) mortality. Colonoscopies are still a highly effective cancer screening tests, experts say, but Canadian researchers have tried to find out what exactly is the effectiveness of colonoscopy in reducing colorectal cancer mortality.

The researchers used a province-wide, administrative database in Ontario, Canada, to identify case patients who received a diagnosis of CRC from 1996 to 2001 and died by 2003. They compared the results of each case with five randomly selected controls, who did not died of CRC.

The procedure, which involves inserting a long and flexible tube in a patient’s large intestine (rectum and colon) to check for ulcers, polyps, tumors and areas of inflammation or bleeding, was linked to fewer deaths from colon cancer only when the tumor or lesion was on the left side of the colon. There was little to no survival benefit to having a colonoscopy if the cancer developed on the right side of the colon, according to the study published in this week’s Annals of Internal Medicine.

The researchers concluded that the screenings probably prevent about 60 to 70 percent of deaths from colorectal cancer, not about 90 percent as previously thought. “When you compare colonoscopy to something like mammography, this is a much better test, but it's not perfect,” said Dr. Nancy Baxter, the study’s lead author and a colorectal surgeon at St. Michael’s Hospital in Toronto.

One possible explanation could be the fact that some colonoscopies that are considered “complete” might not evaluate the entire right colon, according to Baxter.

A recent report released by the American Cancer Society has found that although incidence and mortality rates continue to decrease in both blacks and whites, colon and cancer death rates are now early 50 percent higher in blacks than in whites. According to the report, blacks are less likely to be screened and more likely to be diagnosed after the cancer has spread to other organs. They are also less likely to get the recommended surgical, or radiation treatment after a cancer diagnosis. Colorectal cancer incidence is 14 times higher in adults 50 and older than in people under 50.

The American Cancer Society estimates that 148,810 people will be diagnosed with colorectal cancer this year and 49,960 people will die of the disease. The ACS says the test still prevents most cancers, but most experts agree that patients might want to take great care in selecting their doctors to do their colonoscopies. During the procedure, a tissue sample (biopsy) of the polyp may be taken for lab analysis to determine whether subsequent surgical removal of the tissue is needed. In some cases during colonoscopy, if a polyp or abnormal tissue is found, the doctor may remove it at that time.

A recent study on patients with an average risk of colon cancer has revealed that patients with no signs of precancerous polyps on an initial test have an extremely low risk of developing colon cancer in the next five years.