DISEASE CHARACTERISTICS:

• Meets 1 of the following criteria:

  • Diagnosis of adenocarcinoma of the prostate within the past 6 months

    • Clinical stage T1b-T2b
    • No distant metastases

    • Clinically negative lymph nodes established by imaging (pelvic ± abdominal CT or MRI), nodal sampling, or dissection within the past 3 months

      • Patients with lymph nodes equivocal or questionable by imaging are eligible provided the nodes are ≤ 1.5 cm
      • Lymph node assessment is optional and at the investigator's discretion for patients with Gleason score < 7

    • No evidence of bone metastases on bone scan within the past 3 months

      • Equivocal bone scan findings allowed provided plain films are negative for metastasis
      • Bone metastasis assessment is optional and at the investigator's discretion for patients with Gleason score < 7

    • Baseline serum PSA value performed with an FDA-approved assay within the past 3 months and meets 1 of the following criteria:

      • Gleason score < 7 and PSA < 20 ng/mL
      • Gleason score ≥ 7 and PSA < 15 ng/mL
    • Planning to undergo treatment with either external radiotherapy alone to the prostate ± seminal vesicals only at a dose of 75-79.2 Gy OR brachytherapy alone
    • Pretreatment erectile function as measured by International Index of Erectile Function questionnaire response 3, 4, or 5 on question 1

  • Spouse or partner of these patients (optional)

    • Male or female

PATIENT CHARACTERISTICS:

• Zubrod performance status 0-1
• Serum total testosterone level normal prior to the initiation of radiotherapy
• No myocardial infarction within the past year
• No heart failure within the past 6 months
• No stroke within the past 6 months
• No uncontrolled arrhythmias, hypotension (BP < 90/50 mm Hg), or uncontrolled hypertension (BP > 170/100 mm Hg)
• No known moderate to severe renal insufficiency or end-stage renal disease
• No known severe hepatic impairment
• No prior or concurrent invasive cancer (stage > 0) except for localized basal cell or squamous cell skin carcinoma (stage 0-II), or a hematological malignancy (e.g., leukemia, lymphoma, myeloma) unless disease-free ≥ 5 years
• No known AIDS based upon current CDC definition
• No anatomical genital abnormalities or concurrent conditions that, in the estimation of the physician, would prohibit sexual intercourse or prevent study completion
• No major medical or psychiatric illness that, in the opinion of the investigator, would prevent completion of treatment or would interfere with follow up

PRIOR CONCURRENT THERAPY:

• See Disease Characteristics
• Prior tadalafil allowed
• No prior penile implant or history of bilateral orchiectomy
• No prior prostatectomy, prostatic cryosurgery, high-intensity focused ultrasound, radionuclide prostate brachytherapy, or chemotherapy for prostate cancer
• No prior or anticipated combined external radiotherapy and brachytherapy
• No prior or anticipated external radiotherapy to the pelvic ± para-aortic lymph nodes
• More than 6 months since prior leuteinizing hormone-releasing hormone agonist androgen suppression (e.g., leuprolide acetate, goserelin), antiandrogen (e.g., flutamide, bicalutamide, or nilutamide), or estrogenic (e.g., diethylstilbestrol) agents
• At least 5-7 days since prior use of mechanical (vacuum) devices, intracorporeal, intraurethral, topical, or oral (sildenafil, tadalafil, vardenafil) agents as therapy for erectile dysfunction (ED) or supplements to enhance sexual function
• No other concurrent medical research study that involves the treatment of ED
• No concurrent use of any organic nitrate or as-needed nitrates (e.g., use of nitroglycerin)
• No concurrent cimetidine, ketoconazole, itraconazole, erythromycin, or ritonavir