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.