Co-senior authors Andrew Chan of the Massachusetts General Hospital and Shuji Ogino of the Dana-Farber Cancer Institute, Brigham and Women's Hospital, and Harvard School of Public Health, said both colonoscopy and sigmoidoscopy allow examination of the internal surface of the colon through a fiberoptic tube with a light and camera at the end.
However, while colonoscopy visualizes the entire colon, sigmoidoscopy reaches only the lower third on the left side of the body. Current recommendations for colorectal screening in average-risk individuals age 50 and older are either a sigmoidoscopy every five years, followed by colonoscopy if abnormalities are detected, or a colonoscopy every 10 years, Chan said.
To do this, the investigators analyzed information from almost 89,000 participants in the Nurses' Health Study and the Health Professionals Follow-Up Study, both of which gather comprehensive health information from their participants every two years.
From 1988-2008, the participants reported whether they had either a sigmoidoscopy or colonoscopy in the two preceding years, and whether the examinations were for screening purposes or because of symptoms such as bleeding. For participants who developed cancer or who reported having colon polyps removed, the researchers obtained consent to review their medical records and pathology reports.
Over the 22-year study, 1,815 cases of colorectal cancer and 474 related deaths were documented in the 88,902 study participants.
People who had no cancer or no polyps after either a colonoscopy or sigmoidoscopy had a reduced risk of developing left-side colon cancer, but the same reduced risk was found for people who had polyps removed after either procedure.
However, only a negative colonoscopy was associated with a significant risk reduction in the entire colon, the study said.