282 Background: Ra-223 is an alpha emitter that selectively targets BM. It was shown to improve the survival of patients (pts) with CRPC and BM, and thus is approved as a standard tx for these pts. However, the imaging response during Ra-223 tx is poorly defined. We aimed to describe the imaging response in pts with CRPC and BM treated with Ra-223. Methods: We evaluated the CT and bone scans response among pts with CRPC and BM, who were treated with Ra-223. Tx consisted of an injection administered q 4 weeks up to 6 injections. Scans were done at baseline, after 3 injections, and upon completion of 6 injections. Logistic regression model was used to analyze clinicopathologic factors associated with scans response. Results: 51 pts were included (median age 72). 59% (n = 30) were treated post docetaxel chemotherapy. 47% (n = 24) were treated concomitantly with a systemic standard therapy (e.g enzalutamide or abiraterone). 76% (n = 39) completed the planned 6 injections. A clinical benefit (improvement of sk...