1 Background Persisting residual mass at treatment completion are known in rhabdomyosarcoma(RMS) who have been treated with definitive radiotherapy to the primary site, but their prognostic significance is uncertain. Tumour response as assessed by anatomic imaging is not prognostic and there are only limited studies based on FDG-PET response. We report the prognostic significance of persistent FDG avidity in residual masses, assessed 3-months post completion of radiotherapy, in paediatric RMS who have undergone definitive RT as primary local therapy. 2 Materials and Methods Children≤15 years with Group 3 or 4 RMS treated on a uniform chemotherapy protocol, who received definitive radiotherapy for local control from June 2013-December 2018, and had FDG-PET CT at 3-months post radiotherapy were retrospectively analysed for outcomes. 3 Results Sixty-three children formed the study cohort, (55 Group3 and 8 Group4) FDG-PET CT scan done 3-months post-radiotherapy showed FDG-avid residual mass in 11 patients(17.5%), morphologic only (FDG negative) residual mass in 24 patients(38.1%) and no residual in 28 patients(44.4%). At a median follow-up of 41months (range,10-83months), 3-year Event Free Survival of patients with FDG-avid residual are 45.5% (95%CI:23.8%-86.8%) and for those with morphologic only or no residual are 71.4% (95%CI:59.6%-85.5%). Presence of FDG-avid residual on PET-CT scan 3-months post definitive RT [HR-2.92(95%CI:1.13-7.57),p=0.028] and regional lymph node involvement [HR-3.14(95%CI:1.26-7.78),p=0.014] affected outcomes, which retained significance on multivariate analysis too. 4 Conclusions Persistent metabolic activity in residual disease at the end of therapy in RMS may portend poorer prognosis, and help identify patients who would benefit from alternative treatment strategy.