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"It isn't the weight per se, but there must be some association with increased weight that's making the treatment less effective," said lead researcher Dr. Eric Horwitz, chairman of radiation oncology at Fox Chase Cancer Center in Philadelphia.
"It's not that radiation doesn't work, but it doesn't seem to work as well," he said. "It's still better than not having any treatment."
Being overweight or obese was associated with a small -- 3 percent -- higher rate of prostate cancer relapse and a 7 percent higher rate of cancer spreading. Heavier patients also had a 15 percent increased rate of dying from their cancer and a 5 percent greater rate of dying from other causes, the researchers found.
Obesity among U.S. adults has more than doubled in the past four decades, according to the U.S. Centers for Disease Control and Prevention. While obesity has been linked to certain other cancers, its association with prostate cancer isn't clear, the researchers explained in background notes.
Unlike thinner patients who might be candidates for surgical treatment, Horwitz said overweight and obese men with prostate cancer often have just one option: radiation.
The study was published May 29 in the journal Cancer. It involved 1,442 men, average age 68, treated with radiation therapy for localized prostate cancer between 2001 and 2010. They were followed for an average of four years.
One expert said that numerous theories have been floated to explain the poorer outcomes of obese men with prostate cancer.
"One biological mechanism for the worse survival outcomes among obese men is due to more rapid progression [of the tumor] to distant metastasis" after treatments begin to fail, said Dr. David Samadi, chairman of urology and chief of robotic surgery at Lenox Hill Hospital in New York City.
But while this and other factors might explain the added risk to obese men, "additional research is needed to confirm this relationship," he said.
The researchers showed that even with state-of-the-art radiation therapy, which includes better imaging and higher doses, obesity was associated with worse outcomes, Samadi said.
Dr. Manish Vira, director of the urologic oncology fellowship program at Hofstra North Shore LIJ School of Medicine in Hempstead, N.Y., said, "This study adds to the existing evidence that obesity appears to be associated with increased risk of prostate cancer and more aggressive prostate cancer."
Perhaps additional research will uncover the exact ways obesity leads to more aggressive prostate cancer, he said.
"Based on these and other results, it seems that the next step is to investigate whether enrolling patients in a weight-loss program during and after their prostate cancer treatment could improve patients' chances of being cured of prostate cancer," Vira said.
Horwitz said the lesson from this study could be that patients who are overweight and obese need different treatment.
"Maybe we need to combine treatment, such as radiation and hormone therapy," he said. "Maybe we need to be more aggressive with our treatment."
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SOURCES: David Samadi, M.D., chairman, urology, and chief, robotic surgery, Lenox Hill Hospital, New York City; Eric Horwitz, M.D., chairman, radiation oncology, Fox Chase Cancer Center, Philadelphia, Pa.; Manish Vira, M.D., director, fellowship program, urologic oncology, Hofstra North Shore LIJ School of Medicine, Hempstead, N.Y.; May 29, 2015, Cancer