Quantification of Bone Metastasis of Castration-resistant Prostate Cancer After Enzalutamide and Abiraterone Acetate Using Bone Scan Index on Bone Scintigraphy

Background/aim This study aimed to evaluate the therapeutic effect of enzalutamide (ENZ) or abiraterone acetate (ABI) on bone metastasis in castration-resistant prostate cancer (CRPC), using bone scan index (BSI). Materials and methods Treatment outcomes for 31 patients who had undergone ENZ or ABI treatment were examined for CRPC with bone metastases. Cox proportional-hazards regression models were used to investigate the association between overall survival (OS) and clinical characteristics. Results Median OS after ENZ or ABI treatment was 29 months. Considering the flare phenomenon, BSI in 17 (55%) patients decreased following treatment. In multivariate analysis, low baseline BSI value and a decrease in BSI following treatment were associated with longer OS (hazard ratio [HR]=8.009; p=0.35 and HR=7.025; p=0.045*, respectively). Conclusion Low BSI value before ENZ/ABI treatment and a decrease in BSI following ENZ or ABI treatment are independent predictors of longer OS. BSI could be useful for risk assessment of CRPC patients with bone metastases.