The goal of this clinical research study is to evaluate a system for localizing (targeting for treatment) the prostate for external beam radiation therapy. The system will be used to locate the prostate before treatment and to measure movement of the prostate during treatment. The data collected with the system will be compared to CT images taken during the course of your radiation therapy treatment. Researchers will try to use this information to verify the performance of the system, which could lead to improved positioning and tracking of the prostate during treatment.