Choosing the best test for colon cancer
Nothing elicits a groan from patients as quickly as discussion of uncomfortable screening procedures. Yet those discussions must take place. Colorectal cancer is the third-most common cancer in the United States, with an estimated 150,000 new cases every year. It is also the second leading cause of death by cancer in this country (after lung), with more than 55,000 deaths annually.1 Mortality from colon cancer can be reduced by screening the asymptomatic population. Current recommendations are to start screening in the average-risk population at age 50, using one of the following options2:
- Annual fecal occult blood test (FOBT) or fecal immunochemical test (FIT)
- Flexible sigmoidoscopy (FS) every five years
- Annual FOBT or FIT plus FS every five years
- Double-contrast barium enema every five years
- Colonoscopy every 10 years
For patients whose risk of colon cancer is increased by a family history (first-degree relative) of the disease or colorectal adenoma or by a personal history of long-standing inflammatory bowel disease, the recommended screening test is colonoscopy at an earlier age.
Which test is best?
Screening tests to detect precancerous polyps comprise the primary approach to preventing colon cancer. FOBT has decreased value as a screening test compared with FS or colonoscopy. When FOBT is the chosen screening modality, the present recommendation is to use home rather than in-office FOBT.
The sensitivity of a single unrehydrated FOBT for cancer is approximately 30%; specificity ranges from 90% to 98%. Rehydration increases sensitivity to between 50% and 60%, but it decreases specificity to 90%.3
With advanced colon cancer, FOBT is positive in <25% of patients compared with FS, which identifies 70% of cases; combined use of FS and FOBT detects colorectal neoplasms in 75% of patients screened. Therefore, even the combined strategy misses 25% of cases, compared with colonoscopy.4
Cumulative mortality assessments based on data collected after 18 years of follow-up showed a 33% decrease in colon cancer deaths among persons undergoing annual FOBT vs. those not screened at all.3
The case for sigmoidoscopy
Whether to use sigmoidoscopy or colonoscopy for routine screening is a topic of debate. For patients and clinicians to make the appropriate choice, it helps to understand the pros and cons of each test.
FS permits examination of the rectum as well as the sigmoid and descending colon. The test can be performed with limited bowel preparation by any trained provider. Preparation requires the use of two pre-procedural enemas; sedation is usually not needed. FS is a safe procedure with a very low rate of complications, such as local pain, bleeding, bacteremia, and bowel perforation (one per 25,0005 to one per 10,000 procedures3).
The advantages of FS over colonoscopy are the time required (15 minutes vs. 30-60 minutes, plus the remainder of the day off from work), no need for sedation, easier bowel preparation, lower cost, and lower risk of complications.
FS, followed by colonoscopy in all patients with a detected adenoma, has a sensitivity of 70%.6 FS also reduces mortality attributed to distal neoplasms by 85%-90%.7
Sigmoidoscopy is not without its disadvantages, though. These include the inability to detect proximal adenomas and cancers and the unsatisfactory examination results that can occur with inadequate bowel preparation.
Colonoscopy—for the larger view
Colonoscopy permits examination of the entire colon. A thorough cleansing is required, as well as antibiotic prophylaxis if there is a high risk associated with bacteremia, such as presence of an artificial heart valve or congenital heart defect. The procedure usually requires sedation. Following colonoscopy, the patient is observed and must be escorted home. Normal activities can be resumed the next day.
Complications associated with colonoscopy are perforation (up to two per 1,000 procedures in some series), bleeding, and bacteremia. Screening colonoscopy with removal of neoplastic polyps (adenomas) reduces the incidence of colorectal cancer up to 90% in some studies.
Factors that favor colonoscopy for screening are detection of proximal cancers, cost-effectiveness, and comfort during the procedure. Right-sided neoplasms (i.e., in the ascending or proximal part of the colon) seem to be the most common form in Caucasian and African-American populations older than 70 years. Moreover, proximal cancer in asymptomatic patients is associated with distal lesions such as other neoplasms or polyps in <60% of cases.8 Because it does not access areas of the colon beyond the descending colon, FS will miss those proximal cancers.
Studies conducted in the United States have shown that colonoscopy is cost-effective compared with FS or FOBT plus FS.9 Comparing colonoscopy with FS in terms of comfort, patients undergoing colonoscopy with conscious sedation are less likely to experience periprocedural discomfort and significantly more willing to undergo a subsequent examination.
The primary-care clinician's role Despite evidence that screening tests reduce colorectal cancer incidence and mortality, they are underutilized. Only 34% of U.S. adults older than 50 years have had a sigmoidoscopy or a colonoscopy within the past five years.
Primary-care clinicians should encourage their patients to be tested. When choosing a screening tool, many factors should be considered, including sensitivity, compliance, cost, risks, and acceptability. Patients should understand the available options and be actively involved in making the choice. Although colonoscopy is the most efficacious screening method for colon cancer, some patients will opt for FS because of its acceptability, widespread delivery, and higher sensitivity over FOBT. But any testing is better than none—even FOBT if nothing else is available.
Dr. Surawicz is professor of medicine, assistant dean for faculty development, and chief of gastroenterology at Harborview Medical Center at the University of Washington, Seattle. Dr. Stepan is a visiting physician from the Carol Davila University of Medicine in Bucharest, Romania, currently working in the Pathology Department at the University of Washington, Seattle.
1. Jemal A, Murray T, Ward E, et al. Cancer statistics, 2005. CA Cancer J Clin. 2005;55:10-30.
2. Smith RA, Cokkinides V, Eyre HJ. American Cancer Society guidelines for the early detection of cancer, 2006. CA Cancer J Clin. 2006;56:11-25.
3. Pignone M, Rich M, Teutsch SM, et al. Screening for colorectal cancer in adults at average risk: a summary of the evidence for the U.S. Preventive Services Task Force. Ann Intern Med. 2002;137:132-141.
4. Lieberman DA, Weiss DG, for the Veterans Affairs Cooperative Study Group 380. One-time screening for colorectal cancer with combined fecal occult-blood testing and examination of the distal colon. N Engl J Med. 2001; 345:555-560.
5. Anderson ML, Pasha TM, Leighton JA. Endoscopic perforation of the colon: lessons from a 10-year study. Am J Gastroenterol. 2000;95:3418-3422.
6. Ransohoff DF, Sandler RS. Screening for colorectal cancer. N Engl J Med. 2002;346:40-44.
7. Newcomb PA, Norfleet RG, Storer BE, et al. Screening sigmoidoscopy and colorectal cancer mortality. J Natl Cancer Inst. 1992;84:1572-1575.
8. Imperiale TF, Wagner DR, Lin CY, et al. Risk of advanced proximal neoplasms in asymptomatic adults according to the distal colorectal findings. N Engl J Med. 2000;343:169-174.
9. Sonnenberg A, Delco F, Inadomi JM. Cost-effectiveness of colonoscopy in screening for colorectal cancer. Ann Intern Med. 2000;133:573-584.