Dose escalation in pediatric pelvic Ewing's Sarcoma: Insights from a national treatment planning exercise.

Purpose: The Inter Ewing-1 trial investigates dose escalation in definitive radiotherapy (randomization 54 Gy versus 64.8 Gy). Prior to the trial's initiation, the French Pediatric Radiotherapy Group conducted a national treatment planning exercise to assess dosimetric discrepancies associated with this dose escalation.

Methods and materials: A non-operated pelvic pediatric case of Ewing's sarcoma was selected. CT-scan and delineated structures were distributed to accredited participants for pediatric radio- and/or proton-therapy. Treatment planning included a simultaneous integrated boost: 54 Gy in 30 fractions (1.80 Gy/fraction) for the pre-chemotherapy tumor volume, and 63.9 Gy (2.13 Gy/fraction) for the post-chemotherapy volume. Dose constraints for organs-at-risk and target coverage were provided. All treatment plans were collected, analyzed for deviations and compared. Data were projected in a 21-dimensional(D) space and reduced to 2-D for visual representation.

Results and discussion: Eleven French centers submitted 14 plans (10 photon/4 proton plans). One photon plan exhibited one major and two minor deviations on organ-at-risk constraints. All photon plans presented a minor deviation regarding the constraint D0.1 cc Testis. No significant difference was observed between photons and protons for target coverage or for sparing most critical organs; however, protons showed significant advantages in DmeanBowel (9.9+/-1.1 Gy (photons) vs 3.5+/-0.4 Gy (protons), p < 0.01), and DmeanAnalCanal (3.8+/-0.7 Gy (photons) vs 2.2+/-0.9 Gy (protons), p < 0.01). Data projection revealed 3 clusters: one for protons, one for photons and one with two photon outliers.

Conclusion: This initiative provided valuable insights into the Ewing-1 protocol, demonstrating treatment planning homogeneity among centers along with successful adherence to dose escalation constraints, with protons offering additional benefits as lower integral doses to critical organs.