2183 - Spot-Scanning Proton Arc Therapy for Pediatric Whole Lung Irradiation

Purpose/Objective(s): The aim of this study was to compare the dosimetric outcomes of spot-scanning proton arc (SPArc) therapy with volumetric modulated arc therapy (VMAT) and static intensity modulated proton therapy (IMPT) for pediatric whole lung irradiation (WLI).

Materials/Methods: We identified patients with pulmonary metastases who were consecutively simulated for WLI from 2022 to 2025. ITVs were standardized to encompass adjacent vertebrae, irrespective of patient age, for consistent target delineation. OARs included the heart, cardiac substructures, thyroid, esophagus, and liver. VMAT, 4-field IMPT, and SPArc plans were retrospectively generated in Raystation for each patient with a prescription dose of 15 Gy(RBE) in 10 fractions. SPArc and IMPT plans were robustly optimized with ± 5 mm setup and 3.5% range uncertainties. One-sided Wilcoxon signed-rank tests were used to compare dosimetric parameters for paired plans.

Results: Fifteen plans were created for 5 patients (mean age 16.7 years) with primary Ewing sarcoma, sarcoma with rhabdomyoblastic differentiation, and undifferentiated sarcoma. All plans met target coverage and OAR constraints per standard photon IMRT objectives. SPArc significantly reduced the average mean heart dose to 2.6 Gy(RBE) compared to 5.1 Gy(RBE) for IMPT (p=0.03) and 9.3 Gy(RBE) for VMAT (p=0.03). Similar improvements were observed for all cardiac substructures, thyroid, esophagus, and liver (Table 1). Integral body dose was also lower with SPArc (54.4 J) vs IMPT (69.1 J, p=0.03) and VMAT (108.7 J, p=0.03). ¹Data presented as average mean dose ± standard deviation, Gy(RBE). ²Groups compared using Wilcoxon signed-rank test.

Conclusion: SPArc achieves significant and clinically meaningful improvements in cardiac dosimetry for pediatric patients undergoing WLI. Such minimization of dose to vital surrounding organs particularly the heart and its substructures is crucial in any setting of radiation delivery, but particularly in the pediatric population, which is more vulnerable to late radiation associated morbidities.