LAUSR

Background: Persisting residual masses at treatment completion are known in rhabdomyosarcoma (RMS) treated with definitive radiotherapy (RT) to the primary site, but their prognostic significance is uncertain. Tumor response as assessed by anatomic imaging is not prognostic and studies based on 18F-FDG-PET response are limited. We report the prognostic significance of persistent FDG-avidity in residual masses, assessed 3-month postdefinitive RT, in pediatric RMS. Materials and Methods: Children 15 years old or below with Group III/IV RMS who received only definitive radiotherapy for local control from June 2013 to December 2018, and had 18F-FDG-PET CT at 3 months post-RT were retrospectively analyzed for outcomes and other prognostic factors. Results: Sixty-three children were eligible (Group III-55, Group IV-8). 18F-FDG-PET CT scan done 3 months postradiotherapy showed FDG-avid residual masses in 10 patients (15.9%), anatomic residual in 24 (38.1%), and no anatomic/FDG-avid residual in 29(46.0%). At a median follow-up of 38 months (interquartile range, 24 to 55 mo), 3-year EFS of patients with FDG-avid residual masses was 40.0% (95% CI: 18.7% to 85.5%) versus the rest of the cohort, which was 71.9% (95% CI: 59.8% to 86.5%) (P=0.008). Three-year OS of patients with FDG-avid residual masses was 50.8% (95% CI: 25.7% to 100.0%) versus the rest of the cohort, which was 77.0% (95% CI: 65.1% to 91.0%) (P=0.037). Presence of FDG-avid residual disease persisting post-RT affected both EFS [HR-3.34 (95% CI: 1.29 to 8.68) (P=0.013)] and OS [HR-3.20 (95% CI: 1.01 to 10.12) (P=0.048)] on univariate analysis and this significance was retained for EFS in multivariate analysis [HR-3.52 (95% CI: 1.33 to 9.30) (P=0.011)]. Conclusions: Persistent metabolic activity in residual disease post-chemoradiotherapy in RMS may portend a poorer prognosis with an increased risk of relapse. This subset of high-risk patients needs to be identified, and further trials are warranted to develop strategies to improve their outcomes.