DISEASE CHARACTERISTICS: - Adenocarcinoma of the prostate treated primarily with radical prostatectomy - Pathologically proven to be lymph node-negative by pelvic lymphadenectomy (N0) or lymph node status pathologically unknown (Nx [undissected pelvic lymph nodes because lymph node dissection is not required]) - Any type of radical prostatectomy allowed, including retropubic, perineal, laparoscopic or robotically assisted - Meets 1 of the following pathologic classifications: - T3 N0/Nx disease - T2 N0/Nx disease with or without positive prostatectomy margins - N1 patients are ineligible, as are those with pelvic lymph node enlargement ≥ 1.5 cm in greatest dimension by CT scan or MRI of the pelvis, unless the enlarged lymph node is negative - Prostatectomy Gleason score of 8 or less - A post-radical prostatectomy entry PSA of ≥ 0.1 and ≤ 1.0 ng/mL at least 6 weeks after prostatectomy and within 30 days of registration - Serum total testosterone ≥ 40% of the lower limit of normal (patients who have had a unilateral orchiectomy are eligible as long as this requirement is met) - No distant metastases based on history/physical examination, CT scan or MRI of the abdomen and pelvis, and bone scan - No palpable prostatic fossa abnormality/mass suggestive of recurrence, unless shown by biopsy under ultrasound guidance not to contain cancer PATIENT CHARACTERISTICS: - Zubrod performance status 0-1 - Platelets ≥ 100,000/mm^3 - Hemoglobin ≥ 10.0 g/dL (the use of transfusion or other intervention to achieve this is allowed) - AST or ALT < 2 x upper limit of normal - No prior invasive malignancy (except nonmelanoma skin cancer) unless disease-free for a minimum of 5 years (e.g., carcinoma in situ of the oral cavity is permissible) - No severe, active co-morbidity, including any of the following: - History of inflammatory bowel disease - History of hepatitis B or C - Unstable angina and/or congestive heart failure requiring hospitalization within the past 6 months - Transmural myocardial infarction within the past 6 months - Acute bacterial or fungal infection requiring intravenous antibiotics at the time of registration - Chronic obstructive pulmonary disease exacerbation or other respiratory illness requiring hospitalization or precluding study therapy at the time of registration - Hepatic insufficiency resulting in clinical jaundice and/or coagulation defects - Acquired immune deficiency syndrome (AIDS) based upon current CDC definition - HIV testing is not required for entry - No prior allergic reaction to the study drug(s) involved in this protocol PRIOR CONCURRENT THERAPY: - See Disease Characteristics - No androgen deprivation therapy started prior to prostatectomy for > 6 months duration - No androgen deprivation therapy started after prostatectomy and prior to registration - No prior pelvic radiotherapy