Background: In the Phase 3 ALSYMPCA trial, radium-223 (Ra-223) demonstrated an overall survival (OS) benefit and a favorable safety profile in patients with metastatic castration-resistant prostate cancer (mCRPC). REASSURE (NCT02141438) is a global, prospective, single-arm, observational study of Ra-223 use in patients with mCRPC with bone metastases within routine clinical settings. Utilizing data from the second planned interim analysis, we evaluated safety outcomes in patients with mCRPC treated with Ra-223 following external beam radiation therapy (EBRT) in the US. Methods: In this descriptive analysis (data cutoff 3-20-2019), we included patients who received EBRT to bone prior to Ra-223 (≤2 years prior to Ra-223 first dose); results are presented relative to the overall US subset of patients enrolled into REASSURE. The focus of this abstract is on incidence of hematological toxicities, bone fractures, and second primary malignancies (SPM). Results: Of N=498 patients in the US subset, 118 patients received prior EBRT to bone. Median duration of observation was 11.7 months among patients who received EBRT and 11.9 months for the overall US subset. Any-grade drug-related hematological treatment-emergent adverse events (TEAEs) occurred in 8% (10/118) and 9% (47/498) of patients who received EBRT and the overall subset, respectively (Table). Grade ≥3 drug-related hematological TEAEs occurred in 5% (6/118) and 6% (32/498) of patients who received EBRT and the overall subset, respectively. Bone fractures occurred in 7% (8/118) and 4% (19/498) of patients who received EBRT and the overall subset, respectively. Six SPMs occurred in 5/118 patients who received EBRT (4%), and 11 SPMs occurred in 10/498 patients in the overall subset (2%). Results of bone fracture events by bone health agent use will be included in the full presentation. Conclusions: In this descriptive analysis of real-world data, patients who received EBRT prior to Ra-223 did not demonstrate an increased incidence of hematological toxicities relative to the overall US subset. Although percent of SPMs and bone fractures seems higher among patients treated with EBRT relative to the overall US subset, the real-world incidence of these events remains low.