Colon Polyps, Flexible Sigmoidoscopy and Colon Cancer Screening
Abstracts & Commentary
Synopsis: Patients with an advanced distal adenoma are twice as likely to have an advanced proximal adenoma as patients with a non-advanced distal adenoma.
Sources: Schoen RE, et al, for the Polyp Prevention Trial. Gastroenterology 1998;115:533-541; Ahlquist DA. Gastroenterology 1998;115:777-786.
In this large study performed at multiple centers throughout the United States, Schoen and colleagues sought to evaluate the clinical significance of small tubular adenomas with low-grade dysplasia, found on flexible sigmoidoscopy. The study was performed on 981 subjects who were found to have distal adenomas at colonoscopy before randomization in the Polyp Prevention Trial. Among these, 47% had at least one distal adenoma that was regarded as pathologically advanced, 22% had a proximal adenoma, and 4% had an advanced proximal adenoma. While an advanced proximal adenoma was more likely to be found in a patient with an advanced distal adenoma than in a patient with a non-advanced distal adenoma (5.9% vs 2.9%; P = 0.03), 15 of 42 advanced proximal adenomas would have been missed among patients with a non-advanced distal adenoma if colonoscopy had not been performed. Schoen et al conclude that patients with an advanced distal adenoma are twice as likely to have an advanced proximal adenoma as patients with a non-advanced distal adenoma. However, failing to proceed to colonoscopy in patients with a non-advanced distal adenoma would result in not detecting a sizeable percentage of the prevalent advanced proximal adenomas. They felt that these data supported the performance of colonoscopy in patients with a non-advanced distal adenoma.
The accompanying editorial provides an excellent review of the role of flexible sigmoidoscopy in colon cancer screening, emphasizes several important points, and concludes that, while flexible sigmoidoscopy is an excellent tool for screening recto-sigmoid neoplasms, it is insensitive and ineffective for the detection of proximal colon neoplasms and that consideration should be given to screening techniques that evaluate the entire colon.
Comment by Eamonn M.M. Quigley, MD
The role of screening in the detection of colorectal cancer and in preventing mortality related to this common malignant disease is now established, and protocols for colon cancer screening have been promulgated by several professional organizations, health care providers, and government agencies. Flexible sigmoidoscopy, in particular, has been advocated as playing a central role in national colon cancer screening programs and will undoubtedly be frequently performed by primary care physicians for this reason. This paper and the accompanying editorial address a number of important issues regarding the issue of flexible sigmoidoscopy in colorectal cancer screening. The data from the polyp prevention trial provide insights into the significance of polyps of various histological grade detected at flexible sigmoidoscopy. While previous studies have clearly demonstrated that all polyps visualized on flexible sigmoidoscopy should be biopsied, the clinical interpretation of polyp histology has been somewhat controversial. While nobody would dispute the non-significance of hyperplastic polyps, some have suggested that patients could be selected for full colonoscopy based on the degree of dysplasia within the resected polyp. This study clearly demonstrates that, while those with high-grade polyps within the range of a flexible sigmoidoscopy are more likely to have high-grade polyps more proximally, the presence of "low grade" histology in a distal polyp does not protect against "high grade" proximal polyps. The simple conclusion from this study, therefore, is that all patients found to have an adenomatous polyp on flexible sigmoidoscopy should undergo colonoscopy. Ahlquist’s careful and thoughtful editorial raises several additional issues. He first argues that there is little or no evidence to suggest that flexible sigmoidoscopy is a useful screening tool for proximal colon cancer. In support of this, he points out that, if colonoscopy was based exclusively on the finding of a recto-sigmoid adenoma at flexible sigmoidoscopy, approximately 70-80% of all proximal cancers would be missed. Additionally, by careful analysis of the data from this study, he suggests that, while using sigmoidoscopic findings to triage colonoscopy may increase the likelihood of finding a proximal adenoma on subsequent colonoscopy, it does so at the expense of sharply reducing its already low sensitivity and screening efficacy for proximal neoplasia. The lessons for the physician performing screening flexible sigmoidoscopy appear straightforward. Flexible sigmoidoscopy is a valuable tool for the detection of recto-sigmoid polyps and has an important role to play in the prevention of rectosigmoid carcinoma; however, it does not have a significant effect on the detection or prevention of more proximal polyps or tumors. This begs the inevitable question to which we do not, as yet, have the answer: Should colonoscopy be the screening technique of choice?
References
1. Winawer SJ, et al. Gastroenterology 1997;112:594-642.
2. Bond JH for the Practice Parameters Committee of the American College of Gastroenterology Position Paper. Ann Intern Med 1993;119:836-843.
3. Rex DK, et al. Gastroenterology 1992;102:317-319.
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