Inclusion Criteria

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