Tadalafil in Preventing Erectile Dysfunction in Patients With Prostate Cancer Treated With Radiation Therapy

OBJECTIVES: Primary * To determine whether tadalafil maintains spontaneous (off-drug) erectile function, as measured by the International Index of Erectile Function (IIEF) as compared to placebo at weeks 28-30 after initiation of radiotherapy in patients with prostate cancer. Secondary * Determine the difference in spontaneous (off-drug) erectile function between tadalafil and placebo at 1 and 2 years. * Determine the difference in overall sexual function as measured by the IIEF between tadalafil and placebo at weeks 28-30 and at 1 and 2 years. * Determine differences in patient and partner overall sexual satisfaction as measured by the Sexual Adjustment Questionnaire (SAQ) between tadalafil and placebo at weeks 28-30 and at 1 and 2 years. * Determine differences in patient and partner marital adjustment as measured by Locke's Marital Adjustment Test (LMAT) between tadalafil and placebo at weeks 28-30 and at 1 and 2 years. * Determine associations between patient and partner overall sexual satisfaction as measured by SAQ and patient and partner marital adjustment as measured by LMAT at weeks 28-30 and at 1 and 2 years. * Determine patient-related factors (i.e., age, pretreatment sexual response, tobacco use, and comorbidities) that may predict response to tadalafil therapy at weeks 28-30 and at 1 and 2 years. * Determine the difference in adverse events between tadalafil and placebo as assessed by Common Toxicity Criteria for Adverse Effects (CTCAE) v3.0 criteria. Tertiary * Characterization of preference and erectile function among patients who choose to stay on (or if on placebo, to start) tadalafil, a phosphodiesterase type 5 (PDE5) inhibitor other than tadalafil, a non-PDE5-inhibitor erectile aide, or no PDE5 inhibitor or erectile aide at 28-30 weeks and at 1 and 2 years. * Identification of radiotherapy factors (i.e., modality, prescribed total dose, planning target volume margin, penile bulb dose-volume parameters) associated with erectile function. * Evaluation of the number of patients screened for eligibility, the number eligible that are presented the study, the number who refuse, and the number who are accrued. OUTLINE: This is a multicenter study. Patients are stratified according to age (≤ 65 years vs \> 65 years) and radiotherapy treatment (external beam radiation therapy vs brachytherapy\*). Patients are randomized to 1 of 2 treatment arms. Note: \* Radiotherapy start date for brachytherapy patients is the date of the procedure. All patients undergo either external beam radiotherapy alone to the prostate ± seminal vesicles only or low-dose rate permanent brachytherapy alone. * Arm I: Beginning ≤ 7 days after the start of radiotherapy, patients receive oral tadalafil once daily for 24 weeks in the absence of disease progression or unacceptable toxicity. * Arm II: Beginning ≤ 7 days after the start of radiotherapy, patients receive oral placebo once daily for 24 weeks in the absence of disease progression or unacceptable toxicity. Patients complete the International Index of Erectile Function (IIEF), the Sexual Adjustment Questionnaire (SAQ), the Locke's Marital Adjustment Test (LMAT) (if applicable), and the Expanded Prostate Cancer Index Composite Sexual Medications and Devices Evaluations Supplement questionnaires periodically. Partners or spouses complete the SAQ-Partner, LMAT (if applicable), and IIEF questionnaires periodically. After completion of study treatment, patients are followed at 28-30 weeks, and annually for up to 2 years.