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Colorectal Cancer Screening - Page 2

© David Olle

3. Sigmoidoscopy - This procedure is analogous to colonoscopy, but only the descending colon is examined with a sigmoidoscope. An enema is required as prep, and the descending section is inflated. Sedation is not required. Although the highest incidence of colon cancer occurs in the descending section, the procedure misses about one-half of all colon cancers.

4. Double contrast barium enema - This procedure also requires the evacuation of the colon, and the inflation of the colon. The patient drinks a barium sulfate solution that coats the inner lining of the colon. A technician then takes a series of x-rays. The barium sulfate provides contrast to visualize any polyps that may be present. The procedure costs around $250-500. If polyps are found, a follow- up colonoscopy is required in order to remove them. The procedure only finds about one-third of all adenomas and is beginning to fall out of favor.

Newly Emerging Screening Procedures

1. Computed Tomographic Virtual Colonoscopy - This procedure was developed as a faster, more convenient, and less invasive alternative to conventional colonoscopy. The procedure still requires evacuation and inflation of the colon, although studies are continuing on the possibility of eliminating this requirement. Computed tomography provides a three dimensional view of the colon without the need to introduce a flexible tube in the colon. The procedure does not require sedation, and is faster than conventional colonoscopy. The sensitivity of virtual colonoscopy can rival that of conventional colonoscopy when interpreted by physicians skilled in its use. The virtual procedure may be less sensitive in detecting polyps less than 8 mm in diameter, but normally these small polyps are either harmless or would require many years become cancerous. The procedure costs between $500-2000.

2. Fecal DNA testing - The cancerous state is due to gene mutations in the DNA structure of the cell. Since the colorectal epithelium has a very rapid turnover (every 3 to 8 days), this fact opens up attractive possibilities for testing feces for abnormal DNA structures associated with colorectal cancers. Although colorectal cancers occupy less than 1% of the epithelial surface, mutant DNA derived from tumors may account for 14% or more of the recovered DNA in the feces. This indicates tumor cells are sloughed off at a greater rate from the epithelium than are normal cells.

Cancer cells are notable for having a vast number of genetic mutations. However, in order to be economically feasible, fecal DNA screening must focus on one specific DNA defect. Current research indicates that methylation of the DNA molecule at certain sites is a common defect in colorectal cancer. One study found that testing for methylation at the SFRP2 location provided a sensitivity of 90% in detecting cancer in patients.

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