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Rectal Cancer Risk Increases After Prostate Radiation

In a large retrospective study, men who received radiation therapy for prostate cancer had a 1.7-fold higher risk for developing rectal cancer than did men who underwent surgery alone.

Data from previous studies have been mixed on whether risk for rectal cancer increases after prostate radiation. In this study, researchers evaluated a retrospective cohort of prostate cancer patients, using Surveillance, Epidemiology, and End Result (SEER) registry data from 1973 through 1994. Patients were excluded if they had had previous colorectal cancer, had not survived at least 5 years after prostate surgery or radiation, or had developed colorectal cancer within 5 years of prostate cancer treatment. Colon segments were classified as definitely irradiated (i.e., rectum), potentially irradiated (rectosigmoid, sigmoid, or cecum), or not irradiated (remainder of colon).

A total of 30,552 men received radiation, and 55,263 underwent surgery alone; 1437 patients developed colorectal cancer during a mean follow-up of 9 years. Risk for colorectal cancer was higher in radiation patients, but only for those segments of the colon that were definitely or potentially irradiated. The adjusted hazard ratio for developing rectal cancer after radiation therapy was 1.7 (95% CI, 1.4–2.2).

Comment: This evidence is the most definitive yet to show that prostate irradiation increases risk for rectal cancer (which probably begins to develop at about 5 years after irradiation). The level of increased risk is modest — similar to that associated with a family history of colorectal cancer in a single older first-degree relative. Colonoscopy every 10 years would be appropriate for prostate-radiation patients without other risk factors; but, because the rectum is so accessible, a reasonable approach would be to examine it with flexible sigmoidoscopy at the 5-year point between colonoscopies.

— Douglas K. Rex, MD

Published in Journal Watch Gastroenterology May 10, 2005

Citation(s):

Baxter NN et al. Increased risk of rectal cancer after prostate radiation: A population-based study. Gastroenterology 2005 Apr; 128:819-24.

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