- CRPC patient being considered for systemic therapy or in between lines of therapies (>= 14 days from prior treatment) for metastatic disease. Patient must have evidence of castrate resistant prostate cancer as evidenced by a confirmed rising PSA (per Prostate Cancer Working Group 2 [PCWG2] criteria) and a castrate serum testosterone level (i.e. ≤ 50 mg/dl).