Obesity independently predicts biochemical failure after radiation therapy for localized prostate cancer, according to an article in the August 1 print issue of Cancer.

Researchers at M. D. Anderson Cancer Center found that moderately and severely obese patients had a 99 percent greater risk of developing biochemical failure (an early marker of cancer progression) than other patients. The study also reports that obese patients had a 66 percent increased risk for recurrence or metastasis compared with non-obese patients.

This finding mirrors results from a parallel study by M. D. Anderson researchers, reported last year in Clinical Cancer Research, that found that a history of weight gain or obesity at the time of diagnosis also played a role in how aggressive prostate cancer would become after surgery.

"Together, these studies confirm that a man's body mass index can be a significant factor in how well he fares after standard treatments for prostate cancer," said the lead researcher of both studies, Sara Strom, PhD, an associate professor of Epidemiology.

"The fact that the same association was found among patients with different risk profiles, and who were treated with different therapies, would suggest that poorer outcomes in obese men are not related to differences in treatment as much as to differences in tumor behavior between obese and non-obese men," she added.

Strom suggests that obese prostate cancer patients should be followed more closely after treatment. "When patients and their physicians are uncertain about the need for further therapy, our research indicates that a man's weight should be factored into that decision."

According to Strom, the study is the first to examine the relationship between obesity and prostate cancer progression after primary therapy with external beam radiotherapy, a common treatment option. The researchers sought to determine whether obesity is an independent predictor of biochemical and whether cancer actually progressed among those patients.

Researchers examined the records of 873 patients whose prostate cancer was locally confined and who were treated with radiotherapy at M. D. Anderson between 1988 and 2001. Of these patients, 18 percent were mildly obese and 5 percent were moderately to severely obese. Patients who were obese tended to be diagnosed with prostate cancer at an earlier age than patients who were not obese.

After an average follow-up period of 96 months, 295 patients experienced biochemical failure, and cancer recurred in 127 of these patients.

After adjusting for clinical and treatment variables, body mass index significantly predicted whether a patient would experience both rising prostate-specific antigen level and a return of prostate cancer. For example, biochemical failure occurred more quickly with increased body mass index: an average of 30 months for patients with normal weight versus 26 months for patients who were moderately to severely obese. Researchers also found that when comparing obese patients with non-obese patients, obese men had a significantly higher rate of cancer recurrence.

Strom and her colleagues cannot yet say why excess body mass index contributed to cancer progression or whether losing weight after a prostate cancer diagnosis will make any difference in outcome. "But by knowing this association, we may be able to design rational preventive strategies," she concluded.