Colorectal cancer is the third most common cancer in men worldwide, and the second most common cancer in women. It is the fourth most common cause of cancer death worldwide, accounting for approximately 8% of cancer deaths. However, early detection and surgical removal of colon cancer offers a high potential for curing the disease, with a 93% 5-year survival rate for Stage I cancers. This underscores the importance of screening tools, which are tests to detect early signs of colon cancer in seemingly healthy individuals.
Screening tests for colon cancer
The easiest and most widely used screening test for colon cancer is the fecal occult blood test. Stool samples are placed on a special card that detects the presence of invisible blood in the stool, since colon cancer has a propensity to bleed. However, there are limitations to the test. A number of common noncancerous conditions can cause blood in the stool, including hemorrhoids, ulcers and small outpouchings in the bowel wall called colonic diverticula. The test can also miss the presence of small cancerous polyps (growths) and tumors not actively bleeding.
Colonoscopy is a procedure which is used as both a screening and diagnostic test for colon cancer. A doctor directly visualizes the interior lining of the colon using a small camera on a flexible six foot tube which is advanced throughout the entire colon. This identifies polyps (growths) or cancers arising from the colon wall, and allows for biopsy (sampling) of these growths. Before the test, one to two days of colon preparation are required, consisting of a clear liquid (soft drink, broth) diet and laxative use, typically resulting in frequent stools and diarrhea. This cleans the colon out to allow better visualization. Patients are typically given pain medications and sedatives during the test, and are unable to drive for the next 12 hours.
Current recommendations call for colon cancer screening to begin at age 50, with a yearly fecal occult blood test and a colonoscopy every 5 to 10 years.
Despite the importance of colon cancer screening, only about 50% of individuals comply with the recommendations for colonoscopy. Reasons for noncompliance include fear of the procedure, considerable time commitment and discomfort associated with the procedure or laxative preparation. Conventional colonoscopy is also contraindicated in patients on blood thinners at increased risk of bleeding, and in patients with breathing problems unable to be sedated.
Virtual colonoscopy or CT colonography is a less invasive tool for identifying colon polyps and early cancers. A small tube is inserted only into the lower part of the rectum, to introduce air into the colon to allow all the walls to be better seen. The patient then undergoes a CT (computed tomography) scan, which takes pictures of the colon that are used to create computer generated three-dimensional images. There are advantages to virtual colonoscopy over conventional colonoscopy from a patient’s point of view. The test is less invasive, and does not require sedation. The exam also takes considerably less time than a conventional colonoscopy. These factors seem to add up to a significant difference in patient compliance; in one study, 95% of patients who had undergone both procedures preferred virtual colonoscopy, and 36% stated that they would refuse any further colonoscopy if virtual colonoscopy was not an option.
Virtual colonoscopy without laxatives is as effective as conventional colonoscopy
While virtual colonoscopy is preferred by patients, it still requires a full colon cleansing preparation, which can dissuade many individuals. However, researchers at the University of California, San Francisco and the Massachusetts General Hospital have discovered that this uncomfortable ordeal may be unnecessary. In their study, patients simply followed a low fiber diet for two days, and ingested a substance known as a ‘tagging’ agent. This agent marks any residual stool in the colon and allows it to be digitally removed from the CT images of the colon, permitting clear visualization of the colon walls. The study found that there was no statistical difference in the visualization of polyps 10 mm or larger (the size of polyps most likely to harbor cancer cells) between conventional colonoscopy and laxative-free colonoscopy. The majority of participants in the study also indicated that the laxative-free colonoscopy was their exam of choice.
Researchers hope that this less invasive screening tool will encourage more patients to comply with screening recommendations, and catch early, treatable colon cancers that would otherwise go undetected.