BACKGROUND Benign anal lesions are often considered to cause a predisposition to anal cancer. To reexamine this association, we linked national data on hospital discharge and cancer in Denmark. METHODS After making certain exclusions, we used the Danish Central Hospital Discharge Register to identify 68,549 patients hospitalized with benign anal lesions between 1977 and 1989. Through computerized linkage to the Danish Cancer Registry, all incident cases of epidermoid anal cancer and colorectal cancer among these patients were identified. Follow-up for the occurrence of cancer started the month after the date of the first hospital discharge and continued until the patient died (10.6 percent of the sample), emigrated (0.7 percent), or was lost to follow-up (0.04 percent) or until December 31, 1989 (88.6 percent), whichever came first. RESULTS The median follow-up period was 6.2 years. There were 23 epidermoid anal cancers and 416 colorectal cancers. The overall relative risk of anal cancer (observed vs. expected cases) was 4.4 (95 percent confidence interval, 2.8 to 6.6). The relative risk was 12.0 (95 percent confidence interval, 5.2 to 23.6) within the first year after hospitalization for benign lesions, 4.6 (95 percent confidence interval, 2.3 to 8.3) from one to four years after hospitalization, and 1.8 (95 percent confidence interval, 0.5 to 4.7) five or more years after hospitalization. The risk of colorectal cancer was significantly increased only during the first year after hospitalization (relative risk, 2.6; 95 percent confidence interval, 2.1 to 3.1). CONCLUSIONS There is a strong temporal association between the diagnosis of benign anal lesions and the diagnosis of anal cancer. Although we could not exclude the possibility of a moderate increase in the long-term risk of anal cancer, our data do not support the view that benign anal lesions cause anal cancer.
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