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Objectives: This study aims to evaluate the reliability of the radiographic union scale in tibial (RUST) fractures and modified RUST (mRUST) fractures in pediatric forearm fractures treated with elastic stable intramedullary nail (ESIN) and to investigate the effect of the experience of surgeon, thresholds for union, and delayed union decisions., Patients and Methods: In this retrospective study, radiographic images of 20 patients (10 males, 10 females; mean age 8.6±4.3; range, 4 to 11 years) with forearm fractures treated using ESIN between January 2013 and December 2018 were scored by 20 observers based on the RUST and mRUST scores. The observers scored the radiographs at immediate postoperative period, and at 4-, 8-, and 12-week follow-up. Intra- and interobserver agreement for each cortex, RUST, and mRUST were evaluated using intraclass correlation coefficient (ICC). The Fleiss' kappa (κ) coefficient was used in the agreement between evaluators regarding union decision. Receiver operating curves were created to determine the thresholds for radiographic union and delayed union., Results: Intra- and interobserver reliability of the mRUST score (ICC: 0.84 and 0.79) were slightly higher than that of the RUST score (ICC: 0.80 and 0.72). Pediatric orthopedic and trauma surgeons had slightly higher agreement than the residents and general orthopedists for the total mRUST and RUST scores of the eight-week radiographs. Mean RUST and mRUST scores at the union for all fractures were 10.2±3.4 and 13.0±2.1, respectively. Kappa value for union was moderate (0.74). The total mRUST score had a higher predictive value for union than the total RUST score (area under the curve: 0.986 vs. 0.889). A mRUST score of ≥12 and RUST score of ≥9 were considered as the predictors of union. In addition, a mRUST score of ≤7 and RUST score of <9 were considered as the predictors of delayed union., Conclusion: A moderate agreement for both RUST and mRUST scores was found. However, the agreement for mRUST was found to be slightly higher. Healing and union of forearm fractures treated with ESIN can be reliably assessed using RUST and mRUST.