Assessing response to treatment in patients with advanced prostate cancer is more problematic than in patients with virtually any other solid tumor, because advanced prostate cancer includes bone involvement in more than 90% of patients and nodal and other visceral sites in 20% to 30%. Standard RECIST (Response, Evaluation Criteria in Solid Tumors) scores are not a good measure of therapy response in most prostate cancer patients: For example, the oft-reported prostate-specific antigen (PSA) level is not included in RECIST criteria as a validated surrogate marker of response. This lack of tools presents a unique challenge for investigators and pharmaceutical companies alike in their attempts to develop new agents and to validate them as cli…