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Radiotherapy (RT) for pediatric ependymoma remains challenging due to the young age of the patients and the close proximity of the tumour to critical structures. In Germany, Austria and Switzerland, a Radiotherapy Quality Assurance (RTQA) program was implemented within the SIOP Ependymoma II trial to ensure patient safety and protocol compliance. Pre-trial site approvals and prospective plan reviews for patients in stratum 1 and 2 were performed. Plans were submitted by the treating centres to the reference centres Essen and Leipzig, who evaluated the plans regarding compliance to the study protocol and the German RTQA guideline. Results were categorized as per protocol, acceptable variation, justified unacceptable variation or unacceptable variation, respectively. Unacceptable variations required plan modification and re-review. Between May 2019 and December 2021, 33 patients (median age 5.1 years) from 8 institutions were reviewed. Thirty patients in stratum 1 received conformal RT. Within stratum 2, 3 patients received an additional stereotactic boost. The majority (n=31) of the patients were treated with protons. Of the 33 conformal RT plans, 4 (12%) were reviewed as per protocol, 14 (42%) as acceptable variation, 4 (12%) as justified unacceptable variation and 11 (33%) as unacceptable variation. Six of the unacceptable plans were re-submitted after modification. Four of them were approved after re-submission (acceptable variation=1, justified unacceptable variation=3), whereas 2 plans still had unacceptable variations. Reasons for rejecting plans were target volume delineation (44%), uniformity of target volume (33%) and dose to organs at risk (22%). All stereotactic boost plans were accepted (acceptable variation=1, justified unacceptable variation=2). The high rate of rejected initial RT plans underlines the importance of a rigorous RTQA program. Improving the quality of RT plans potentially impacts favourably on outcome and long term side effects although requiring a high effort. [ABSTRACT FROM AUTHOR]