Long-Term Locoregional Control and Pattern of Recurrences After Implementation of Highly Conformal Flank Target Volumes and Image-Guided Intensity Modulated Arc Therapy for Pediatric Wilms Tumors

Purpose: Excellent outcomes in patients with Wilms tumors (WT) have allowed efforts to reduce toxicity and treatment burden while sustaining effectiveness. Concerning radiation therapy, “conventional” flank target volumes combined with 2 opposing photon beams are now being replaced by highly conformal flank target volumes combined with image-guided intensity modulated arc therapy, allowing better sparing of the organs-at-risk, but outcome descriptions based on large cohorts are scarce.Previously, a cohort of 36 patients with excellent locoregional control at 2 years from diagnosis using “modern” flank irradiation has been described. This study aims to investigate locoregional control outcomes, patterns of recurrence, and toxicity in an extended national cohort of patients with WT with a longer follow-up (FU) period. Methods and Materials: Newly diagnosed patients with a WT, irradiated on the flank between January 01, 2015 and August 08, 2025 using highly conformal target volumes, as defined by the International Society for Pediatric Oncology - Renal Tumor Study Group consensus statement, combined with IMAT, were eligible for analysis. The 4-year locoregional control rate (LCR), disease-free interval, event-free survival, overall survival, and acute and estimated late toxicity were assessed. For each locoregional recurrence, image co-registration and dose reconstruction were performed to classify each recurrence as “infield” if ≥99% of the gross tumor volume at recurrence received ≥95% of the prescribed dose at initial irradiation (V95%_{recurrence-IMAT}, ≥99%), “marginal” (V95%_{recurrence-IMAT}, 20.0%-98.9%), or “outfield” (V95%_{recurrence-IMAT}, ≤19.9%). Results: Seventy patients received postoperative flank irradiation only (n = 50) or combined with metastatic site irradiation (n = 20). With a median FU of 4.7 years, the estimated 4-year LCR, disease-free interval, event-free survival, and overall survival were 93.7%, 86.4%, 83.7%, and 93.7%, respectively. Four patients developed a locoregional recurrence, of whom one (marginal recurrence) could have been better covered by the conventional approach. Nausea ± vomiting requiring antiemetics developed in 34 of 63 (54%) patients. In 24 of 70 patients (34%), major constraint violations, mainly involving the pancreatic tail and spleen, were observed. Conclusions: This extended national cohort of patients with WT confirms the excellent LCR, using modern flank irradiation, even after a long-term FU.