The optimal frequency for colonoscopic screening is unknown. When screening is repeated, even immediately after initial colonoscopy, a small percentage of adenomas are found, representing new or previously missed lesions. The Prostate, Lung, Colorectal, and Ovarian cancer screening trial (PLCO) is a randomized, community-based study evaluating the effectiveness of cancer screening tests. In this subinvestigation, Schoen and colleagues examined the yield of flexible sigmoidoscopy three years after an initially negative examination.

More than 154,000 adults 55 to 74 years of age who participated in the PLCO trial were included in this study. Flexible sigmoidoscopy examination results were categorized as follows: (1) abnormal suspicious, when any polypoid mass was found; (2) inadequate, designating insufficient depth of insertion or inadequate bowel preparation; (3) abnormal not suspicious, signifying any abnormality other than a mass or polyp; and (4) negative, indicating no polyps or abnormalities. Data were collected on the basis of location and size of any lesion and pathology reports.

Of the 11,583 patients without a polypoid mass or lesion initially, 9,317 (80.4 percent) returned for repeat screening after three years. Of this group, 1,292 (13.9 percent) had a repeat flexible sigmoidoscopy that was classified as abnormal suspicious, indicating that a polyp or mass was detected. The latter varied in size and number, with most repeat examinations revealing one lesion, and most lesions being smaller than 0.5 cm. Diagnostic follow-up was available for 951 (73.6 percent) of these patients. Overall, 25.2 percent of patients with a distal adenoma had an advanced distal adenoma. Of the 9,317 returnees, 292 (3.1 percent) had an adenoma or cancer. The yield for an advanced adenoma or cancer in the distal colon was 78 (0.8 percent). The yield for adenomas and advanced adenomas in the proximal colon was lower than that for the distal colon. Only 14 (19.4 percent) of the diagnoses of advanced distal adenoma could be attributed to increased depth of insertion or better preparation at the follow-up examination.

Risk factors for finding distal adenomas or advanced distal adenomas included male sex and current smoking. Family history of colorectal cancer in a first-degree relative and inadequate baseline examination also were associated with the finding of advanced distal adenomas. A decreased risk of any distal adenoma was associated with flexible sigmoidoscopy, colonoscopy, or barium enema in the three years before the study.

The incidence of distal adenoma or cancer three years following a negative flexible sigmoidoscopy was 3.1 percent. Of those with advanced distal adenomas, 80.6 percent had lesions that were adequately examined at the initial sigmoidoscopy.

The authors conclude that more frequent examinations will detect lesions and prevent subsequent morbidity, suggesting that colonoscopy alone every 10 years may be insufficient for detecting distal lesions in time to prevent substantial morbidity and mortality. Screening with flexible sigmoidoscopy at more frequent intervals, especially in patients 50 to 65 years of age who are at lower risk for proximal lesions, may be preferable to colonoscopic examination alone every 10 years.