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Radiation for Prostate Cancer May Make Rectal Cancer More Likely

Overall risk for rectal tumors remains low, however

FRIDAY, April 1, 2005 (HealthDayNews) -- Men with prostate cancer typically receive one of two recommended therapies: surgical removal of the organ or radiation to kill cancer cells within the prostate.

Now a new study has found that one type of radiation therapy, called external beam radiation, may trigger an almost 70 percent rise in the risk for rectal cancer.

However, the study's lead author stressed that because rectal cancer is relatively rare, the overall chance of someone getting it remains extremely low, even if the man has received external beam radiation.

"The risk is high enough that people should be aware of it," said Dr. Joel E. Tepper, a professor and chair of the Department of Radiation Oncology at the University of North Carolina at Chapel Hill. "But I don't think it's high enough that it should have an impact on people's decision as to what treatment to get."

Doctors use two types of radiation therapy to treat prostate cancer. One is external beam radiation and consists of radiation directed at the prostate from outside the body. One problem with external beam radiation is that healthy parts of the body receive radiation along with the cancer.

The other form of radiation treatment is brachytherapy, for which tiny radioactive seeds are placed in the prostate, releasing the dose of radiation precisely where it is needed.

As reported in the April issue of Gastroenterology, Tepper and his team collected data on men with prostate cancer who participated in the national Surveillance, Epidemiology and End Results (SEER) registry from 1973 through 1994.

More than 30,500 of the men received radiation, and just over 55,000 underwent surgery. Colorectal cancers developed in 1,437 men.

"In patients who received radiation therapy for prostate cancer, there was an increase in the incidence of rectal cancer," Tepper said. "Presumably, [these cancers were] secondary to the radiation."

According to the North Carolina team, combining the effects of prostate cancer-linked radiation with the age-related risks for rectal cancer already present in older men increases a person's overall risk for rectal cancer by about 70 percent.

"This effect is of approximately the same order of magnitude as having a first-degree relative with colorectal cancer, or with a 10-year increase in age," the researchers noted.

Tepper speculated that because the radiation fields are more confined in brachytherapy, there would be less risk of developing rectal cancer than with external beam radiation.

He stressed, however, that rectal cancer remains relatively rare. According to the American Cancer Society, 40,000 men and women in the United States contract the disease each year. Because of these numbers, Tepper doesn't think this finding should change the treatments for prostate cancer.

"It should have very little implication for what a patient decides to do for treatment," he said. "The relative risk of developing a rectal cancer from the radiation is still quite small -- about one percent. It is, therefore, dwarfed by the implications of the initial cancer that is being treated."

It is important, however, that men treated with radiation get more intensive screening after their treatment to be sure any new cancers are not developing, he said.

An expert from the American Cancer Society, Dr. Durado Brooks, said the North Carolina findings are convincing but apply largely to men who received radiation therapy from the 1970s to the 1990s.

"External beam prostate radiation, at least as done in the '70s through the early '90s, does seem to increase the risk of rectal cancers," said Brooks, the Cancer Society's director of prostate and colorectal cancers.

Brooks is not certain how relevant these findings are for the type of narrow beam external radiation commonly used today, but he believes it still has "some relevance."

"Even with the newer modes of external radiation, there is still going to be some exposure of rectal tissue," he said.

As far as brachytherapy is concerned, Brooks said it's impossible to tell from this study if there is any increased risk of rectal cancer. "It depends on the radiation dose and how much of that spills over to rectal tissue," he explained.

But Brooks said it's important that prostate cancer patients, particularly those who received external beam radiation 10 or more years ago, be aware that the treatment carries with it a substantially increased risk for rectal cancer.

"Most of these men are in the age group that they should be being screened for colorectal cancer regularly, but this is one more thing to encourage them to make sure they are being screened," he said.

More information

The American Cancer Society has more information about prostate cancer.

SOURCES: Joel E. Tepper, M.D., professor and chair, Department of Radiation Oncology, University of North Carolina School of Medicine, Chapel Hill, N.C.; Durado Brooks, M.D., director, prostate and colorectal cancers, American Cancer Society, Atlanta; April, 2005, Gastroenterology
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