RATIONALE: Radiation therapy uses high-energy x-rays and other sources to damage tumor cells. Giving radiation therapy in different ways may kill more tumor cells.

PURPOSE: Phase II trial to study the effectiveness of external-beam radiation therapy followed by implanted radiation therapy in treating patients who have prostate cancer.

OBJECTIVES: I. Determine the acute and late grade 3-5 genitourinary (GU) and gastrointestinal (GI) toxic effects of external beam radiotherapy followed by permanent source brachytherapy using interstitial iodine I 125 seeds in patients with intermediate risk, clinically localized adenocarcinoma of the prostate. II. Determine the freedom from prostate specific antigen (PSA) failure, overall survival, disease specific survival, and local and/or distant clinical relapse rates in patients treated with this regimen.

OUTLINE: This is a multicenter study. Patients undergo external beam radiotherapy (EBRT) to the pelvis 5 days a week for 5 weeks. Permanent source brachytherapy comprised of ultrasound-guided implantation of iodine I 125 seeds into the prostate is performed 2-6 weeks after completion of EBRT. Patients are followed at 1, 3, 6, 9, and 12 months, then every 6 months for 2 years, and then annually thereafter.

PROJECTED ACCRUAL: A total of 110 patients will be accrued for this study within 1 year.

DISEASE CHARACTERISTICS: Histologically proven locally confined adenocarcinoma of the prostate Clinical stage T1c-T2b, NX or N0, M0 Gleason score 2-6 AND PSA greater than 10 ng/dL but no greater than 20 ng/dL OR Gleason score 7 AND PSA no greater than 20 ng/dL Prostate volume no greater than 60 mL by transrectal ultrasound prior to external beam radiotherapy American Urologic Association voiding symptoms score no greater than 18 No distant metastases

PATIENT CHARACTERISTICS: Age: Over 18 Performance status: Zubrod 0-1 Life expectancy: Not specified Hematopoietic: Not specified Hepatic: Not specified Renal: Not specified Other: No hip prosthesis No other malignancy within the past 5 years except basal cell or squamous cell skin cancer No major medical or psychiatric illness that would preclude study Fertile patients must use effective contraception

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No prior chemotherapy Endocrine therapy: Prior induction hormonal therapy allowed only if started no more than 6 months prior to study Radiotherapy: See Disease Characteristics No prior radiotherapy to pelvis Surgery: No prior transurethral resection of prostate No radical surgery for carcinoma of the prostate