ATLANTA (AP) -- Being overweight hurts men's chances of having successful radiation treatment for prostate cancer, according to a study released Monday.

The study by researchers at the University of Texas M.D. Anderson Cancer Center is being called the first to examine the link between obesity and prostate cancer progression after radiation treatment.

Researchers found that moderately and severely obese men had a 70 percent higher risk of having a tumor recur after radiation treatment than thinner men did.

The same researchers last year looked at men who had surgery for prostate cancer, and found that heavier men were more likely to have rising levels of PSA, a blood protein that can signal prostate cancer, after treatment than thinner men were.

"Together, these studies confirm that a man's level of obesity can be a significant factor in how well he fares after standard treatments for prostate cancer," said Sara Strom, the epidemiologist who led the research on both studies.

It's not clear how being overweight affected the success of prostate cancer treatment. Fat tissue, by secreting certain hormones, may somehow have helped the cancer to progress later, she said.

The technology of radiation treatment has been improving, and it's possible that fat tissue in men once contributed to targeting errors that no longer occur as often, the study said.

Whatever the mechanism, the study gives men another reason to stay trim, said Dr. Len Lichtenfeld, the American Cancer Society's deputy chief medical officer.

"It certainly suggests people who are overweight and obese do not do as well," he said.

Strom's study is being published in the Aug. 1 issue of Cancer, a scientific journal published by the society.

The study looked back at the records of 873 prostate cancer patients treated at M.D. Anderson, in Houston, between 1988 and 2001. About 18 percent of them were mildly obese, and 5 percent were moderately to severely obese.

Obesity was characterized by body mass index, a statistic that incorporates height and weight. Under BMI, a man 5 feet, 10 inches tall would be considered normal weight at or below 184 pounds. He would be overweight at 185 to 209 pounds, mildly obese at 210 to 244 pounds, and moderately and severely obese at higher weights.

All of the men in the study had external-beam radiotherapy as a primary treatment for prostate cancer. For years afterward, their health was monitored using digital rectal exams and a blood test that measures prostate specific antigen, or PSA.

Moderately and severely obese men had almost twice the risk of developing elevated PSA levels, Strom said.

Copyright 2006 The Associated Press. All rights reserved.