ASCO PROSTATE: Radiation for Rectal Cancer May Protect Prostate

KISSIMMEE, Fla., Feb. 22 - Men who undergo external beam radiation treatment for rectal cancer appear to have a long-term reduced risk of prostate cancer.

Analysis of data from 1,574 men who were given radiation for rectal cancer found that the rate of prostate cancer was about 70% less than the expected rate in the general population, said Karen Hoffman, M.D., of Massachusetts General Hospital in Boston.

The men, all treated for rectal cancer from 1988 through 1997, were identified by analyzing data from the Surveillance, Epidemiology, and End Results (SEER) registry. The average age of the patients at the time of rectal cancer treatment was 71, Dr. Hoffman reported at a prostate cancer symposium here.

Twenty of these men were subsequently diagnosed with prostate cancer, "a number significantly less than would be expected in the general population of that age and race," she said.

The observed/expected ratio for prostate cancer was 0.28 for men who underwent radiation for rectal cancer. By comparison, the observed/expected ratio for prostate cancer as 0.94 among 3,125 men who had rectal cancer but had no radiation and 1.08 for 24, 578 men diagnosed with colon cancer who were not treated with radiation.

Dr. Hoffman noted that clinical studies have indicated that incidental irradiation of the prostate occurs during other treatments, and such treatments tend to result in lowered prostate specific antigen levels for several years.

"Rectal and prostate cancers tend to be diagnosed at a similar decade of life," she commented. ""Inadvertent irradiation of the prostate during treatment for rectal cancer may sterilize or cyto-reduce subclinical prostate cancer or precancerous intra-epithelial lesions, or may impede prostate cancer development by altering the local hormonal or stromal milieu."