RATIONALE: Radiation therapy uses high-energy x-rays to damage tumor cells and may be an effective treatment for prostate cancer.

PURPOSE: Phase I/II trial to study the effectiveness of radiation therapy in treating patients who have previously untreated stage II or stage III prostate cancer.

OBJECTIVES: I. Determine the maximum tolerated dose of 3-dimensional conformal radiotherapy to the prostate gland and immediately surrounding tissues in patients with stage II or III adenocarcinoma of the prostate. II. Determine the normal tissue toxicity rate of this regimen in the rectums and bladders of these patients. III. Determine local control by clinical and pathologic examination and by PSA determinations in patients treated with this regimen. IV. Determine the distant failure and overall survival of patients treated with this regimen.

OUTLINE: This is a dose escalation study. Patients are stratified according to 1 of the following 3 prognostic groups: Group 1: T1b-c or T2a-b with combined PSA/Gleason (CPG) score 15 and under Group 2: T1b-c or T2a-b with CPG score more than 15, or any T2c with PSA less than 70 ng/mL Group 3: T3 with prostate specific antigen (PSA) less than 70 ng/mL Patients receive 3-dimensional (3-D) conformal radiotherapy 5 days a week for 7.6, 8.2, or 8.8 weeks. Cohorts of 120-270 patients receive escalating doses of 3-D conformal radiotherapy until the maximum tolerated dose (MTD) is determined. The MTD is defined as the dose preceding that at which 20% of patients experience dose-limiting toxicity. Patients are followed every 3 months for 1 year, every 4 months for 1 year, every 6 months for 3 years, and then annually thereafter.

PROJECTED ACCRUAL: A total of 549-801 patients (120-204 patients for the first dose level, 237-321 patients for the second dose level, and 192-276 patients for the third dose level) will be accrued for this study within 4.75 to 5.25 years.

DISEASE CHARACTERISTICS: Previously untreated adenocarcinoma of the prostate T1b-c or T2a-b with a Gleason score more than 5 and PSA greater than 4 ng/mL OR Any T3 PSA under 70 ng/mL within 3 weeks prior to entry and at least 10 days after prostate biopsy No biopsy proven regional nodal involvement No distant metastases

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: Karnofsky 80-100% Hematopoietic: White blood cell (WBC) count of at least 4,000/mm3 Platelet count at least 100,000/mm3 Hemoglobin at least 11 g/dL Hepatic: Not specified Renal: Not specified Other: No major medical or psychiatric illness that would preclude study No other malignancy within the past 5 years

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No prior or concurrent chemotherapy Endocrine therapy: Prior hormonal therapy allowed Radiotherapy: No prior pelvic irradiation Surgery: No prior prostatectomy or cryosurgery for prostate cancer