Bone scan index can be a useful biomarker of survival outcomes in patients with metastatic castration‐resistant prostate cancer treated with radium‐223

Background Bone metastasis of prostate cancer is associated with pain and reduced overall survival (OS). Radium‐223, which is expected to reduce bone pain and prolong OS, was recently approved in Japan. Aim The aim of this study was to investigate the usefulness of the bone scan index by comparing the outcomes and factors according to response in Japanese patients treated with radium‐223. Methods and results Twenty patients receiving radium‐223 were divided into two groups according to whether they did or did not achieve a bone scan index decrease of at least one point (beneficial and non‐beneficial groups, respectively). The clinical characteristics at baseline and after three and six treatment cycles were compared using χ 2 tests and Student's t‐tests or Mann–Whitney U tests, and survival was estimated and compared using the Kaplan–Meier method and log‐rank test, respectively. Fourteen (70%) and six patients (30%) were categorized into the beneficial and non‐beneficial groups, respectively. Patients in the former group were significantly more likely to have a higher Eastern Cooperative Oncology Group performance status score and receive a greater number of radium‐223 injections ( P  < 0.05). Furthermore, patients in the beneficial group had a significantly longer OS (P < 0.05). Regarding safety, one and three patients in the beneficial and non‐beneficial groups, respectively, prematurely discontinued radium‐223 because of an increased prostate‐specific antigen level, decreased hemoglobin level, or femoral fracture. Conclusion Radium‐223 appears generally safe in this population. Patients with good bone scan index response have better performance status, receive more injections of radium‐223, and achieve OS prolongation. Bone scan index is a useful biomarker of survival outcomes and can be a valuable assessment tool in patients with metastatic castration‐resistant prostate cancer who are treated with radium‐223.