For decades now, doctors have urged patients hitting their fifties to get a colonoscopy once every five to ten years to check for colon cancer – a tumor in the large intestine. These are done by gastroenterologists, doctors who specialize in diseases of the digestive tract. In this procedure, a long, flexible viewing scope is inserted at the anus and is extended out progressively longer – further up into the colon – during the exam. Patients are under mild sedation, waking a short while later with no recollection of discomfort, perhaps just slightly sore. A simpler procedure for such a check is the sigmoidoscopy, which uses a shorter scope; sedation isn’t necessary for this exam. Although not seeing beyond the first third of the colon, the majority of colon tumors occur in that portion. Hence a sigmoidoscopy is still a valuable exam, but is not as thorough as a colonoscopy.
During either procedure, the doctor is looking for tumors but also for less serious growths called polyps. These are outgrowths of cells that may be precancerous or may be benign (non cancerous). The most common shape for a polyp is a bit like a mushroom, with a ‘stalk’ coming out of the intestinal wall and a roundish growth at the end of it. There are also flat polyps that bulge up slightly but are wider and have no stalk, hence being harder to see. To a doctor doing a colonoscopy, a stalked polyp sticks out like the proverbial sore thumb compared to flat ones. Whenever found and whenever possible, polyps are removed, since some will be precancerous. This isn’t known until after a pathologist looks at a sample under the microscope. Patients with a precancerous polyp will be recommended to have another procedure in three years, rather than the five to ten for those with no polyps or only a benign polyp.
Either stalked or flat polyps may be precancerous or benign, yet recent evidence suggests the protruding kind have a lower chance of turning malignant (~ 10%) compared to the flat variety. For the latter, the risk of being cancerous goes up with size: The smallest flat polyps have only a 5% chance of being malignant, but that chance rises to 23% for the largest. This makes flat polyps worrisome, given that even the larger ones are harder to see than the stalked variety. Also, for reasons not understood, they are most numerous in the portion of the colon furthest from the anus. This means they will be detected only by a colonoscopy, not a sigmoidoscopy. There have always been small subsets of patients who develop colon cancer despite having had a colonoscopy within the previous five years. The most likely cause of these cases was a precancerous flat polyp that was missed during examination.
Doctors have become well aware of the significance of flat polyps and their detection rate is going up. Improvements in the endoscopes used for colonoscopy is also helping. The traditional endoscope uses a full-spectrum white light at the exploratory end. Doctors realized that while such a light is best for everyday usage in homes and offices, it might not be the best for colonoscopies. They are now increasingly using “narrow band” light sources that leave out the red wavelengths within white light, giving off a blue-green glow. Once a doctor gets used to this type of light, polyps can be seen much better with it. If you have ever looked through colored cellophane or two-color 3-D glasses, you know that objects with the same color of the cellophane appear colorless. Similarly, small red blood vessels that outline polyps can be better seen with light not containing any red wavelengths.
Flat polyps are also harder to remove because of their flat nature. Polyps with a stalk are literally pinched off inside the colon with a snare at the end of the endoscope; this is harder with the flat type. In these cases a patient is scheduled for follow-up endoscopy at a more specialized facility that is prepared to perform minor surgery to remove flat polyps. People who get periodic colonoscopies have much lower rates of colon cancer than those who skip this exam. As doctors get better at detecting flat polyps, the rate of patients developing colon cancer within five years of a colonoscopy is expected to drop. In other words, as doctors learn more about flat polyps and as their instruments are improved, colonoscopy is becoming even more of a life-saver than it always has been.