Background: The Functional Assessment of Cancer Therapy-Radionuclide Therapy (FACT-RNT) was recently developed as the first patient-reported outcome (PRO) measure designed to monitor RNT-relevant symptoms and toxicities for patients with advanced metastatic castrate-resistant prostate cancer (mCRPC). This study aimed to examine preliminary reliability of the FACT-RNT in patients receiving RNT as part of standard of care. Methods: Patients scheduled to start 177Lu-PSMA-617 for advanced mCRPC December 2022 – August 2023 who were fluent in English or Spanish completed the FACT-RNT on day 1 of each RNT cycle administered at UCLA. The FACT-RNT is a 15-item measure assessing symptoms and toxicities on a 0-4 scale; total scores range from 0-60, with higher scores indicating better quality of life. Reliability was assessed in R using data from cycles 1-4 with Cronbach’s alpha test of internal consistency, Guttman’s Lamba 6 test of split-half reliability, inter-item correlations, and test-retest reliability. Results: Patients (N=43) were, on average, 69 years old (SD = 9, range = 51-91) at RNT cycle 1, were mostly non-Hispanic (79%) and White (58%) with Gleason score ≥ 7 (67%) at diagnosis. Patients had undergone surgery (35%), chemotherapy (98%), radiation (65%), hormone therapy (100%), and/or immunotherapy (28%). Some patients who recently began therapy were not yet due for follow-up cycles, and 9 (21%) died during the study period. Completion of the FACT-RNT ranged from n=43 (Cycle 1) to n=19 (Cycle 4). The FACT-RNT total score ranged from 18-60 across cycles. Internal consistency reliability ranged from 0.82 (good) to 0.91 (excellent), and split-half reliability ranged from 0.92 to 0.99 (excellent). Average inter-item correlations were acceptable for scaling purposes (all exceeding 0.20). Within-subject correlations between scores were r=0.75 for cycles 2 vs. 3 and r=0.94 for cycles 3 vs. 4, indicating good measurement stability across cycles. Conclusions: In preliminary analyses, the FACT-RNT demonstrates good to excellent internal consistency across the first four cycles, moderate to strong consistency of measuring a common underlying construct, and good measurement stability. Findings support reliability of the FACT-RNT and its use in future research.