Men who get radiation treatment for prostate cancer are 70% more likely to develop rectal cancer than those who are treated with surgery only, according to a new study in the journal Gastroenterology (Vol. 128, No. 4:819-824).

The increased risk appears to be caused by the radiation that hits the rectum when the prostate is treated, according to researchers from the University of Minnesota and the University of North Carolina.

Men with prostate cancer should discuss this risk with their doctor when deciding on their treatment, said lead researcher Nancy Baxter, MD, PhD, and be closely monitored for rectal cancer if they do choose radiation. Baxter is a colorectal surgeon at the University of Minnesota Medical School and Cancer Center.

Other experts agreed.

"Men who have been irradiated need to be particularly careful that they are getting screened for colorectal cancer," said Durado Brooks, MD, director of prostate and colorectal cancer for the American Cancer Society. "But the vast majority of these men are in the age group where they should be getting screened anyway."

ACS advises most people to begin colorectal cancer screening at age 50. Most of the men in the study were 60 or older.

Prostate cancer is the most common cancer among men in the US, except for skin cancer, striking more than 230,000 men each year. Nearly all of them survive 5 years beyond their diagnosis, and around 92% survive at least 10 years. That makes it especially important to understand the long-term effects of treatment, Baxter and her colleagues wrote.

They studied more than 85,000 prostate cancer patients diagnosed between 1973 and 1994. All of the men had survived at least 5 years after their prostate cancer diagnosis (experts believe it takes at least that long for a radiation-induced cancer to develop), and had no personal history of colorectal cancer.

About 30,000 of the men had been treated with external beam radiation, and about 55,000 treated with surgery only. The researchers compared the risk of these men developing cancer in the rectum, which would receive the most radiation exposure during prostate therapy; in the lower part of the colon, which might receive some radiation from prostate therapy; and in the upper part of the colon, which would be unlikely to receive much radiation.

They found an increased risk only for cancers in the rectum. Radiation therapy did not affect cancers in other parts of the colon.

Does this mean men should forget about getting radiation therapy if they develop prostate cancer?

"The findings of our study do not suggest prostate cancer treatment should change," Baxter said.

Even with the added risk from radiation, the overall rate of rectal cancers in the radiation group was still low, she noted.

And newer radiation techniques may not have the same effect.

"Treatment has changed since [the study was completed]," said Brooks. "Now men get intensity modulated radiation therapy or 3-dimensional conformal radiation therapy to locate exactly where the prostate is sitting and focus radiation intensely in that area."

Though the rectum may still be exposed to some radiation, he explained, those new techniques probably reduce the amount -- and the risk.

Moreover, the study did not include patients treated with brachytherapy, a technique that implants radioactive seeds directly into the prostate, so it's not known if this technique may have an effect on rectal cancer risk.

Brooks said men should choose their prostate cancer treatment after weighing all the side effects of each option. That goes even for men who may already be at high risk of colorectal cancer because of a family history or other factors.

"The potential increased risk [of rectal cancer] is one small piece that should go into the overall consideration," he said.

Above all, though, men who have had prostate radiation should get screened for colorectal cancer regularly and be alert to any symptoms of the disease (like blood in the stool), Brooks said.