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The findings could have implications for men trying to decide which treatment is best for their prostate cancer, the researchers said.
"We saw an increased risk for lung, bladder and rectal cancer among patients treated with external beam radiation for prostate cancer," lead researcher Dr. Naeem Bhojani, from the University of Montreal's Department of Urology, said during a Monday teleconference at the American Urological Association annual meeting, in Orlando, Fla.
"However, in absolute terms, the absolute risk associated with the development of secondary malignancies in patients exposed to external beam radiation therapy is quite small," Bhojani said.
Among the treatments for prostate cancer are surgical removal of the prostate, called radical prostatectomy; implanted radioactive seeds that deliver radiation specifically to the prostate gland; and external beam radiation, which is a non-surgical procedure that delivers radiation to the prostate from outside the body.
Bhojani said external beam radiation is probably a better treatment choice for older patients rather than younger patients with longer life expectancies, who may be at risk for developing these secondary cancers.
For the study, by researchers from Canada, Italy and the United States, Bhojani's group collected data on 10,333 men being treated for localized prostate cancer. Among these men, 6,196 had their prostate surgically removed, and 4,137 underwent external beam radiation.
The researchers then looked at the later development of bladder, lung and rectal cancer, to see which treatment was more likely to increase the risk of these malignancies.
They found that men who'd undergone external beam radiation had a threefold increased rate of bladder cancer, a 1.8-fold increased rate of lung cancer, and a 1.7-fold increased rate of colorectal cancer, compared with men who underwent radical prostatectomy.
One cancer expert said he couldn't explain why external beam radiation would increase the risk of these cancers, especially lung cancer.
"I'm not sure how external beam radiation to the pelvis for prostate cancer would increase the risk for lung cancer," said Dr. Bruce Roth, professor of medicine and urologic surgery at Vanderbilt University.
The radiation therapy would be more likely to increase the risk for bladder cancer and colorectal cancer, which are nearer the prostate, he noted.
Roth said it's important to give prostate cancer patients several treatment choices. "I will give them a number of options and let them pick based on the side effect profile. There are many patients who may not be good surgical candidates, therefore you might lean toward external beam radiotherapy," he said.
In another study presented Monday, researchers found that survivors of childhood cancers treated with the chemotherapy drug cyclophosphamide were five times likelier to develop bladder cancer later in life, compared with the general population.
Cyclophosphamide is an immunosuppressive drug used to treat several types of cancers and is known to cause bladder damage that can lead to bladder cancer.
In patients followed for up to 50 years, the researchers found that the chances of developing bladder cancer increased over time. The percentage of patients diagnosed with bladder cancer at 20, 30, 40 and 50 years of age was 0.02 percent, 0.08 percent, 0.37 percent and 0.71 percent, respectively, the researchers reported.
SOURCES: Bruce Roth, M.D., professor, medicine and urologic surgery, Vanderbilt University, Nashville, Tenn.; May 19, 2008, teleconference with Naeem Bhojani, M.D., University of Montreal, Quebec, Canada; May 19, 2008, presentation, American Urological Association annual meeting, Orlando, Fla.
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