Data from the Geneva Cancer Registry show an increased long-term risk of colon cancer in men who have undergone external radiation therapy for prostate cancer.

“The risk of second cancer after irradiation, although probably small, needs nevertheless to be carefully monitored,” the study team advises.

Dr. Christine Bouchardy from the University of Geneva, Switzerland and colleagues analyzed data on 1,134 men diagnosed with prostate cancer between 1980 and 1998 who survived for at least 5 years after diagnosis. Of these, 264 were treated with external radiation.

During follow-up through the end of 2003, 19 men out of the total group developed colorectal cancer.

The risk of colorectal cancer among the men who did not have radiation therapy was not increased compared to the general population, but it was 3.4-times higher than normal among the men who did have radiation, the team reports in the International Journal of Cancer.

On further analysis, the risk was significantly increased for colon cancer specifically but not for rectal cancer.

The risk of colon cancer was mainly elevated in the 5- to 9-year period after diagnosis, according to Bouchardy and colleagues.

They say “this serious long-term side effect should be discussed” with patients in weighing the pros and cons of radiation therapy for treating prostate cancer.

SOURCE: International Journal of Cancer, September 1, 2008.