Small adenomas detected during fecal occult blood test screening for colorectal cancer: the impact of serendipity

Article Abstract:

The fecal occult blood test (FOBT), a test performed to detect blood in the feces, is currently recommended for people over the age of 50. A positive FOBT may be indicative of cancer or large adenomas (benign tumors, greater than one centimeter in diameter) that can be surgically removed. Approximately 30 percent of the adenomas detected by the FOBT are small adenomas (less than one centimeter), the most common size. Since people with small adenomas are considered at risk for subsequent tumors, including malignant ones, detection of these lesions with FOBT is considered advantageous. When small adenomas are found, periodic colonoscopy (examination of the large bowel under strong illumination) is recommended after their removal. However, false-negatives (an incorrectly negative test result) and false-positives (an incorrectly positive test result, possibly caused by diet or bleeding from a cause unrelated to a tumor) can occur with FOBT. Serendipity plays an important role in FOBT results. A 'lucky hit' is said to occur when a nonbleeding adenoma (which would not normally show up on FOBT) tests positive because of other factors. In fact, serendipity is a frequent cause of detection, since relatively few small adenomas bleed. This is demonstrated by calculating several false-negative, false-positive, true-negative, and true-positive probabilities. The best way to reduce the contribution of serendipity to FOBT results is to increase the test's specificity (number of true negatives divided by the sum of the true negatives and false positives). This means reducing the number of false positives, possibly by restricting the patient's diet or medication prior to testing. Another issue is the actual importance of detecting small adenomas during FOBT. The removal of all such growths, most of which do not progress to cancer, would be inefficient. Questions may also be raised about the need for repeated surveillance colonoscopy after adenomas are removed, and about the effectiveness of this procedure. Aggressive surveillance of a condition discovered by chance contains a built-in paradox. (Consumer Summary produced by Reliance Medical Information, Inc.)

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Colonic polyps, occult blood, and chance

Article Abstract:

Fecal occult blood testing (FOBT) is a simple, inexpensive way to test for tumors of the large bowel (colon) and rectum. In addition to malignancies, the method reveals benign growths (adenomas, or polyps) as well. Since many colorectal cancers arise from adenomas, early detection of these growths can reduce the cancer rate. Approximately 20 percent of 50- to 59-year-olds and one-third of those over 70 who test positive with FOBT have adenoma. However, an article in the July 4, 1990 issue of The Journal of the American Medical Association claims that most small (less than one centimeter) adenomas detected with FOBT are detected by chance, and that the value of routine follow-up of these patients with colonoscopy should be questioned. The adenoma rate for individuals with a positive FOBT is similar to the adenoma rate for the general population. Most of these lesions do not bleed, or do not bleed enough to be detected by FOBT. The authors of this article question whether removal of such polyps is essential. Although this conclusion is not necessarily supported, colonoscopic surveillance may be performed too often in people with such conditions. Most small adenomas will not progress to cancer. Only 8 percent of untreated polyps larger than one centimeter become malignant within 10 years. However, can these legitimately be ignored, which would be the result if FOBT were de-emphasized? The US Preventive Services Task Force and the Canadian Task Force on the Periodic Health Examination concluded that there is not enough evidence to recommend FOBT for people at average risk for colorectal cancer, but is there not enough evidence to stop such testing altogether. Better tests would help solve the dilemma, as would improved understanding of how these cancers develop. (Consumer Summary produced by Reliance Medical Information, Inc.)

editorial

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Sigmoidoscopic screening in the 1990s

Article Abstract:

Routine sigmoidoscopic screening may help reduce the number of deaths from colorectal cancer significantly. This type of screening can be used to detect small colorectal cancers or adenomatous polyps. Small colorectal cancers or adenomatous polyps can be removed surgically before development of a large, incurable cancer. A research study found that this type of screening may reduce the number of deaths from colorectal cancer. Another research study found that routine screening reduced the number of deaths among patients who had undergone removal of adenomatous polyps. This type of screening should be performed on individuals between 50 and 75 years old every three to five years. Individuals who have had an adenomatous polyp removed have a higher risk of developing colorectal cancer and should undergo screening more often than other individuals.

Usage, Diagnosis, Colorectal cancer, Sigmoidoscopy

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