How to improve the implementation of new radiotherapy guidelines? Interests and limits of a national training workshop.

OBJECTIVES: Ewing sarcoma radiotherapy requires precise target volume delineation. As part of the implementation of the SIOP Inter-Ewing-1 trial in France, we conducted a national benchmark and educational workshop to assess and improve delineation quality among pediatric radiation oncology centers. METHODS: French pediatric radiotherapy centers were invited to participate in a benchmark case of postoperative clavicular Ewing sarcoma. Twelve radiation oncologists submitted initial delineations, and eight participated both before and after an educational workshop. Delineations were evaluated qualitatively using overlap color maps and quantitatively using Dice coefficient (DC), Hausdorff distance (HD), concordance/discordance indices, and volume ratios, relative to an expert-defined reference contour (CRef). RESULTS: Pre-workshop delineations showed substantial heterogeneity, with all DC values <0.7 except for CTVp_Pre, and HD values >20 mm for all target volumes. The most frequent deviation was under-contouring of initial bone involvement, which was the key point of discussions during the workshop. After the workshop, all target volumes increased, reflecting improved inclusion of areas at risk for relapse. Significant improvements were observed for DC in GTVp_Post and CTVp_Post, as well as for concordance indices. However, discordance indices increased for most volumes due to target overestimation, and HD did not significantly improve. CONCLUSIONS: This national benchmark revealed poor delineation homogeneity in postoperative Ewing sarcoma case. The educational workshop provided modest improvements but variability remained substantial. ADVANCES IN KNOWLEDGE: These findings highlight the critical need for prospective radiotherapy quality assurance (RTQA), including prospective individual case review, to enhance compliance with contouring guidelines and ensure high-quality radiotherapy delivery.