Colorectal cancer is the second-leading cause of cancer-related death in the U.S. Fortunately, it is preventable through proper screening. In a September 10 article published in the Huffington Post, two radiologists, Dr. Joel Bortz and Dr. Joseph Lebovic, argue that CT colonography, or "virtual" colonoscopy, is superior to traditional colonoscopy for colon cancer screening. They contend that the only reason colonoscopy is endorsed as a preferred screening method by organizations such as the American Cancer Society and the American College of Gastroenterology is because of the "lucrative" nature of the procedure.
A review of the medical literature shows why colonoscopy has been considered the gold standard for colorectal cancer screening and why it should be recommended for all patients eligible to undergo the test.
Colonoscopy can prevent colorectal cancer
Colonoscopy is one of the few screening procedures that is proven to save lives. In a landmark study from Memorial Sloan Kettering Cancer Center, colonoscopy with polyp removal was shown to decrease mortality from colorectal cancer by 53%. Colonoscopic polypectomy also reduces the incidence of colorectal cancer. Since most colon cancers develop from polyps, colonoscopy can prevent the development of cancer through the detection and removal of lesions that might ultimately turn malignant.
No study in the medical literature demonstrates a similar effect for CT colonography because polyp removal can only be accomplished with traditional colonoscopy. In fact, both the Centers for Medicare and Medicaid Services and the U.S. Preventative Services Task Force concluded that there was insufficient evidence to recommend CT colonography for colon cancer screening.
One prep, one procedure
The worst part of colonoscopy or CT colonography is the bowel preparation. Although progress has been made in developing a laxative-free technique for CT colonography, a full bowel preparation is required currently for both colonoscopy and CT colonography. With the development of low-volume, split-dosed preps for colonoscopy, tolerance has improved.
So, what happens if a polyp is suggested by CT colonography? According to the radiology literature, a small polyp measuring 6-9 mm in size may be followed with a repeat CT scan in three years. Larger polyps should be removed immediately, which in most cases requires referral to another specialist for a second bowel preparation and a second procedure.
If colonoscopy is performed as the primary screening method, polyps can be removed at the initial exam with a single bowel preparation.
With proper technique, colonoscopy is a safe and comfortable test
Colonoscopy is typically performed with sedation, which requires patients to miss a day from work and can be a source of complications related to the procedure. However, procedural sedation, with either propofol or combination sedatives, is generally well tolerated and offers patients the opportunity to undergo a pain-free examination. Additional techniques, such as carbon dioxide insufflation and warm water immersion, have been shown to improve patient tolerance as well.
Although CT colonography is touted as a more comfortable procedure, approximately 7% of patients undergoing the test experience abdominal pain due to air required to properly image the bowel. Study results vary regarding patient preferences for screening, but one large multicenter study demonstrated that satisfaction was greatest with colonoscopy, including less embarrassment, less pain and more willingness to repeat the procedure when compared to CT colonography and barium enema.
Colonoscopy is better at detecting polyps, large and small
A rigorous multicenter study, published in the New England Journal of Medicine, demonstrated that CT colonography missed polyps measuring 6-9 mm in 22% of patients, and larger polyps measuring 10 mm or greater were missed in 10% of patients. All of these lesions were subsequently detected at colonoscopy. Although miss rates for colonoscopy have been published, the use of CT colonography as a sole screening method is worrisome because CT imaging cannot detect flat polyps, which may grow faster than raised polyps and are more often associated with malignant transformation. Colonoscopy permits both detection and removal of these dangerous lesions.
Radiation risk is no small matter
While imaging techniques and equipment differ across the country, each CT scan of the abdomen delivers a radiation exposure of between 7 and 8 mSv. This is estimated to result in approximately 150 cases of radiation-related cancer per 100,000 patients screened by CT colonography every five years from the age of 50 to 80.
More frequent scanning would be required if small polyps are found. Leaving polyps in place is one of the most controversial recommendations of those who support CT colonography as an alternative to colonoscopy, not only because small polyps can grow into cancer but also because serial CT scans would subject otherwise healthy patients to even greater risks of radiation-associated injury over a lifetime.
Colonoscopy is now a covered health benefit for all Medicare patients and, in most states, for commercially-insured patients. The Affordable Care Act stipulates that coverage for colonoscopy be included in all health plans starting on or after September 23, 2010.
However, not every patient is a candidate for colonoscopy. Patients with severe cardiac or respiratory disease or advanced age might be at increased risk for complications from sedation, and patients with anatomical abnormalities of the colon could be at higher risk for bowel injury during colonoscopy. For these patients, radiologic studies, like barium enema and CT colonography, offer another option for screening. The recent approval of a fecal DNA test provides a non-invasive alternative for patients who decline or are not candidates for colonoscopy.
Among all screening options, colonoscopy remains the preferred choice for most patients because of its superior accuracy, tolerability, safety, and the ability to remove polyps and prevent cancer in a single procedure. While the debate about colonoscopy vs. CT colonography might go on, colonoscopy will always be required when signs of a precancerous polyp or early colon cancer are detected.
Of course, all decisions about the most appropriate screening test to undergo should be conducted with the guidance of your physician.