According to a new study by researchers in the U.S. colonoscopies do not detect all colorectal cancers; they say that flat lesions can be easily overlooked during a colonoscopy.
The researchers say flat, nonpolypoid colorectal tumours are common and may develop into cancer compared to those most routinely diagnosed type of colorectal polyps.
But they say as flat lesions are hard to find because they merge with surrounding healthy tissues, they are easily overlooked during a colonoscopy.
The scientists from the Veterans Affairs Palo Alto Health Care System, say the risk of colorectal cancer is high for patients who have flat lesions.
Their study examined data from 1,819 patients, almost all men with an average age of 64, from July 2003 to June 2004 undergoing elective colonoscopy to find out how common the flat lesions were and their link with colorectal cancer.
The group were divided into three groups; screening, surveillance and a group of patients with symptoms for colorectal cancer.
Flat lesions were found in 170 people (10%) and the surveillance group had the highest incidence with more than 15% who had flat lesions; approximately six percent of the screening and symptoms groups had flat lesions.
The researchers found that the flat growths were five times more likely to be cancerous than polyps.
Colorectal cancer, also called colon cancer or large bowel cancer, includes cancerous growths in the colon, rectum and appendix.
Dr. Roy M. Soetikno and his colleagues found that the flat and depressed type of lesions were almost 10 times more likely to be cancerous than polypoid lesions and they say they are more difficult to detect by colonoscopy or computed tomography colonography.
Dr. Soetikno says because the lesions are only subtly different from those of the normal surrounding membrane they are are challenging to detect.
Colorectal cancer is the third most common form of cancer and the second leading cause of cancer-related death in the western world.
A colonoscopy is a common and very safe procedure that examines the lining of the lower intestinal tract called the colon or large intestine using a flexible tube that has a light and miniature TV camera on the tip.
Colonoscopy is the only procedure that visualizes the entire colon and allows for the detection and removal of lesions and polyps before they turn into cancer.
The researchers say the study highlights the importance of quality colonoscopy screening.
The study is published in the current issue of the Journal of the American Medical Association.