Background:The imaging response to radium-223 therapy is at present poorly described. We aimed to describe the imaging response to radium-223 treatment.Methods:We retrospectively evaluated the computed tomography (CT) and bone scintigraphy response of metastatic castration-resistant prostate cancer (CRPC) patients treated with radium-223, in eight centers in three countries.Results:A total of 130 patients were included, the majority (n=84, 65%) received radium-223 post docetaxel. Thirty-four of 99 patients with available data (34%) received concomitant abiraterone or enzalutamide. A total of 54% (n=70) patients completed the planned six injections of radium-223. In patients with available data, a transient increase in bone metastases-related pain was observed in 27% (n=33/124) and an improvement of bone metastases-related pain on treatment with radium-223 was noted in 49% of patients (n=61/124). At 3 and 6 months of treatment with radium-223, bone imaging showed stable disease in 74% (n=84/113) and 94% of patients (n=93/99) with available data, respectively. An increase in the number of bone lesions was documented at 3 months compared with baseline in 26% (n=29/113) and at 6 months compared with 3 months in 6% of patients (n=6/99), respectively. Radiological extraskeletal disease progression occurred in 46% of patients (n=57/124) with available CT data at 3 and/or 6 months.Conclusions:Progression of bone metastases during radium-223 therapy is uncommon. A bone flare (pain and/or radiological) may be noted during the first 3 months, and should not be confused with progression. Imaging by CT scan should be considered after three and six doses of radium-223 to rule out extraskeletal disease progression.