HOLLYWOOD, FLA. — Radiation treatment of prostate cancer boosted men's risk of later developing rectal cancer by 70%, according to data collected in a United States cancer registry kept by the National Cancer Institute.
But the absolute risk of rectal cancer remained low in men who got radiation therapy, about 0.1%/yr, and so does not warrant curtailing radiation therapy as an option for treating prostate cancer, Nancy N. Baxter, M.D., said at a symposium on gastrointestinal cancers sponsored by the American Society of Clinical Oncology.
Instead, the finding means that men who receive radiation treatment for prostate cancer should undergo regular surveillance for rectal cancer by either sigmoidoscopy or colonoscopy starting 5 years after their radiation therapy, said Dr. Baxter, a surgeon at the University of Minnesota in Minneapolis.
Surveillance should begin even in men who are not old enough to otherwise qualify, she told this newspaper. In these men, visual surveillance once every 5-10 years would probably be adequate. Since all of the excess colorectal cancers would be in the rectum, digital rectal examinations also might be a useful surveillance method.
During radiation therapy of the prostate, the anterior wall of the rectum is always radiated; other parts of the colon receive minimal or no radiation. The study was done to assess the impact of this exposure, she explained.
The study used data collected in the Surveillance Epidemiology, and End Results registry, which has collected follow-up information on annual samples of cancer patients since 1973. Dr. Baxter focused on patients with invasive, nonmetastatic, adenocarcinoma of the prostate who entered the registry during 1973-1994, a total of more than 85,000 men. The group included 30,552 men who had radiation treatment and 55,263 who were treated surgically.
The analysis excluded men who developed any form of colorectal cancer during the first 5 years following their prostate cancer treatment because of the low likelihood that these cancers were caused by the radiation therapy. This left 1,437 men who developed colorectal cancer during an average follow-up of 9 years; 267 of these were rectal cancers.
The unadjusted data showed an excess of rectal cancers among men who had radiation treatment. However, because colorectal cancers are much more common in older men, a multivariate analysis was done that controlled for several demographic variables including age, Dr. Baxter said at the symposium, also sponsored by the American Gastroenterological Association, the American Society for Therapeutic Radiation and Oncology, and the Society of Surgical Oncology.
In the proportional hazards model, radiation treatment was associated with a statistically significant, 70% increase in the rate of rectal cancer. Radiation therapy led to no significant rise in the rate of cancer in the rectosigmoid region, cecum, or any other part of the colorectal tract, she said.
The increased risk is roughly comparable with the added risk conferred when men age by 10 years, or among men who have one first-degree relative with colorectal cancer.
The absolute rate of rectal cancer during a 10-year period was about one case/200 men among those who had their prostate cancer managed surgically, compared with a rate of about one case/100 men among those treated with radiation.