Am Fam Physician. 2002;66(3):501-505
Early detection can reduce mortality from colorectal cancer, currently the second leading cause of cancer deaths in the United States. Unfortunately, each of the available screening techniques has significant disadvantages. Colonoscopy is currently limited by patient discomfort, incomplete examination in up to 15 percent of cases, and missing up to 24 percent of polypoid lesions. Virtual computed tomographic colonography (VCTC) uses spiral computed tomography (CT) data to generate images in two and three dimensions producing endoscopic-like examination of the colon plus the ability to examine extraintestinal lesions in the abdomen and pelvis. Laghi and colleagues compared VCTC with colonoscopy in screening for colorectal lesions in 165 patients at high risk for colorectal cancer.
Most of the 79 men and 86 women were referred for colonoscopy because of a positive fecal occult blood test (31 percent) or altered bowel habits (24 percent). Other reasons for referral included history of colorectal resection (22 percent), rectal bleeding (15 percent), unexplained anemia (4 percent), or history of polyps (3 percent). Patients with inflammatory bowel disease, familial polyposis, acute abdominal symptoms, or pregnancy were excluded. Patients underwent VCTC before colonoscopy. The average time of scanning was 20 minutes, and interpretation took an average of 25 minutes. Data were interpreted by two experienced gastrointestinal radiologists working independently. Differences between the radiologists were resolved by joint review of images and development of consensus. Colonoscopy was performed within four hours of VCTC. The endoscopist was not informed of the VCTC findings, and the VCTC reviewers were unaware of the colonoscopy findings on each patient.
Colonoscopy detected 30 cancers and 37 polyps. All cancers were identified and localized exactly by VCTC. Colonoscopy was not completed in nine patients, but VCTC did not detect any lesions in these cases. Of the 37 polyps, VCTC detected 29, for a sensitivity of 78 percent. The ability of VCTC to detect polyps was dependent on size, with 92 percent sensitivity for those 10 mm or larger in diameter but only 50 percent sensitivity for those 5 mm or smaller. Six polyps were reported by VCTC but not confirmed by colonoscopy. These false positives were attributed to hypertrophic plicae, fecal debris, or perceptual error.
Overall, the per-patient sensitivity and specificity of VCTC were about 92 and 97 percent, respectively, compared with colonoscopy. Discomfort during VCTC was rated as low by 68 percent of patients and by 7 percent during colonoscopy. Conversely, high discomfort was reported by 32 percent during colonoscopy and by 2 percent during VCTC. Other lesions detected by VCTC included renal cysts and stones, gallstones, abdominal aortic aneurysms, liver metastases, and lymphadenopathies.
The authors conclude that VCTC provides diagnostic accuracy comparable to colonoscopy for lesions larger than 6 mm in diameter in patients at high risk of colorectal cancer. They point out that several issues, especially cost and radiation exposure, must be resolved before results can be applied to the general population.