Of 111 carcinomas developing in 73 patients with ulcerative colitis, 46 (41·5%) arose in the rectum where they are directly accessible to sigmoidoscopy. Fifty-eight per cent of single carcinomas developed in the rectum. The extent and frequency of rectal dysplasia was assessed by examining slides of rectal mucosa with an eyepiece micrometer. Slides from 46 patients with carcinoma and 22 patients with dysplasia but no carcinoma in whom proctectomy or proctocolectomy had been carried out were examined by this method. Thirteen of 15 patients with carcinoma of the colon (87%) and 21/22 patients (95%) with large bowel dysplasia showed evidence only of rectal dysplasia. However, there was marked variability in the proportion of dysplastic rectal mucosa even in those patients with rectal carcinoma, while in some patients dysplasia was limited to a small focus. Because of the possibility of false negative biopsies due to sampling error, multiple biopsies should be taken to detect dysplasia. Their state should be recorded and deliberately varied at subsequent visits. Careful sigmoidoscopy and multiple biopsies in this study had potential value as an aid in the detection of 85-90% of all carcinomas. In practice the figure would almost certainly be lower due to intrinsic bias (see discussion) so that, although regular sigmoidoscopy and biopsy would be of great value when colonoscopy is not available, the latter should be included in any long-term programme of carcinoma prevention.
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