Enzalutamide + External Beam Rt For Prostate

* The participant will be given a study drug-dosing diary for each of 6 treatment cycles. Each treatment cycle lasts 28 days (4 weeks), during which time the participant will be taking the study drug enzalutamide by mouth (4 pills daily). The diary will also include special instructions for taking the enzalutamide. * The participant will also be undergoing external beam radiation therapy during the study, for a duration of 8 ½-10 weeks. Treatment will be administered on an outpatient basis. This part of the study is considered standard of care. * The participant will be required to have fiducial markers placed within the prostate as part of this study. These are very small gold markers that are placed in the prostate. This procedure is similar to the biopsy that the participant had to diagnose their cancer. This procedure is routinely performed to permit imaging and position corrections to improve the precision of the external beam (radiation) delivery (Image Guided Radiation Therapy). The placement of the three gold markers is performed by an Urologist. The fiducials are placed under ultrasound guidance. This is also considered standard of care and is not experimental. * Planned Follow-up: Participants will be followed on study at a 3 month follow-up after treatment (cessation of enzalutamide). Additional follow-up to assess the status of the participant's cancer will be conducted at the discretion of the treating physicians as part of the participant's routine medical care.