Radiation sometimes affects the ability for a person to have a normal erection. Complete loss of erections after radiation treatment can happen in 40-50% of treated patients. There are medications, like sildenafil (also known as Viagra), that can help the ability to get back erections in almost 70% of such patients.

The purpose of this study is to see if taking Viagra every day starting right before, during and for about 6 months after treatment, could reduce the risk of long-term erectile dysfunction.

This protocol is a randomized prospective study comparing prophylactic and on demand Sildenafil Citrate usage administered during and after radiotherapy in combination with or without hormone therapy versus radiotherapy alone for the preservation of erectile function in potent patients with clinically localized prostate cancer. The primary objectives of this protocol are to determine if prophylactic and on demand usage of Sildenafil Citrate can preserve spontaneous erectile function as well as diminish the time to restoration of erectile function in potent men receiving radiotherapy with or without hormone therapy for localized prostate cancer. Secondary objectives of this study are: to determine whether prophylactic and on demand use of Sildenafil Citrate during therapy may improve the long-term response to the drug following treatment; to determine whether prophylactic and on demand use of Sildenafil Citrate during therapy will reduce acute and late urinary effects of radiotherapy; to determine the impact of dosimetry and radiation exposure to both the neurovascular bundle and bulb of the penis on erectile function preservation in the study population undergoing brachytherapy and hormone therapy; to asses the impact on quality of life following prophylactic and on demand Sildenafil Citrate usage in the study population; to determine the relationship between testosterone levels, erectile function and efficacy of prophylactic and on demand Sildenafil Citrate usage in the study population.

Inclusion Criteria:

• To receive external beam radiation therapy and/or brachytherapy for biopsy-proven prostate cancer.
• Baseline International Index of Erectile Function (IIEF) erectile function domain score >= 17.
• Patients receiving radiotherapy with hormone therapy undergoing a maximum of 9 months of hormone therapy will be acceptable.

Exclusion Criteria:

• Baseline IIEF-EF (1-30) domain score of <17 pre-therapy
• Current routine use of erectogenic agents (use of agents > 4 times per month would constitute "routine use")
• Neoadjuvant androgen deprivation therapy started more than 1 month prior to entry into study (intake of Bicalutamide alone does not constitute exclusion from study entry)
• Have a clinically significant penile deformity in the opinion of the investigator (ie: Peyronie's Disease)
• Non-organ confined disease
• Prior prostate surgery or cryotherapy
• Prior prostate radiotherapy started more than 2 weeks prior to entry into study
• Currently taking 0.8mg Flomax daily
• Penile implant history
• Present at baseline with chronic angina requiring nitrates, angina occurring during sexual intercourse or unstable angina within the last 6 months.
• History of myocardial infarction, coronary artery bypass graft surgery or percutaneous coronary intervention within 90 days of baseline visit.
• History of significant cardiac conduction defect within 90 days of baseline visit.
• Exhibit systolic blood pressure > 170 or < 90 mm Hg or diastolic blood pressure > 100 or < 50 mm Hg at baseline visit or have a history of malignant hypertension
• Have any condition that would interfere with the subject's ability to provide informed consent or comply with study instructions, would place subject at increased risk, or might confound the interpretation of the study results
• Currently receive treatment with nitrates, cancer chemotherapy, or antiandrogens (except finasteride taken as Propecia or Proscar)
• Have a history of drug, alcohol, or substance abuse within the past 6 months, as assessed by the investigator.