Isoflavones and Radiation Therapy in Treating Patients With Localized Prostate Cancer

OBJECTIVES: * Determine the effect of soy protein isolate (isoflavones) and radiotherapy on the modulation of biomarkers in patients with localized prostate cancer. * Determine the toxicity of this regimen in these patients. * Determine the effect of this regimen on the quality of life of these patients. OUTLINE: This is a randomized, placebo-controlled study. Patients are randomized to 1 of 2 treatment arms. * Arm I: Patients receive oral soy protein isolate (isoflavones) twice daily for 6 months in the absence of disease progression or unacceptable toxicity. * Arm II: Patients receive an oral placebo twice daily for 6 months in the absence of disease progression or unacceptable toxicity. In both arms, patients also undergo radiotherapy while receiving isoflavones or placebo. Quality of life is assessed periodically. PROJECTED ACCRUAL: A total of 70 patients will be accrued for this study.