Prostate cancer is the 2nd most common cancer in men worldwide. Based on the results of several recent clinical trials, systemic treatments including hormone inhibitors, docetaxel, cabazitaxel, and PARP inhibitors are being used as standard treatment for patients with metastatic castration-resistant prostate cancer. However, there is insufficient research on salvage therapy for patients with metastatic castration-resistant prostate cancer who have failed standard treatment. In this study, the investigators will evaluate the effectiveness and safety of ifosfamide in castration-resistant prostate cancer by analyzing the treatment outcomes of patients who received ifosfamide/mesna treatment as salvage therapy.