Ra-223 was approved because it was shown to improve the length of the lives of the men with prostate cancer who received it. Despite that important benefit, it is not known to improve other standard markers of prostate cancer such as PSA blood tests (a blood marker that is used to track cancer activity in men who have prostate cancer) and standard imaging scans such as bone scans and computed tomography (CT) scans. If participants and their doctors do not have good markers of whether or not the cancer is responding to therapy, it is harder to make decisions about whether to continue that therapy. This is a current problem.