Pencil-beam scanning proton therapy is particularly sensitive to anatomic changes, which may negatively affect plan quality. We hypothesized that magnetic resonance imaging (MRI)-based adaptation can improve plan quality for pediatric patients. By evaluating 230 MRI scans acquired in treatment position during proton therapy, we found that onefourth of patients exhibited anatomic change, more than half of which result in suboptimal delivered plans. This highlights the potential role for MRI-guided proton Purpose: Pencil-beam scanning proton therapy is particularly sensitive to anatomic changes, which may affect the delivered dose distribution. This study examined whether offline adaptation using on-treatment magnetic resonance imaging (MRI) scan during proton therapy could improve plan quality for pediatric patients. Methods and Materials: Pediatric patients with at least 1 MRI scan in the treatment position (MRI tx ) during proton therapy between January 2017 and July 2019 were retrospectively reviewed. Patients underwent MRI and computed tomography simulation. Cases were planned with scenario-based optimization with 3 mm/3% positional/ range uncertainty. Patients demonstrating anatomic change on MRI tx were recontoured. The original plans were applied to the anatomy-of-the-day for dose recalculation (delivered plans). Plans were subsequently reoptimized offline, using original beam angles and dose-volume constraints (adapted plans). Delivered plans were compared with adapted plans to detect significant changes in plan quality, defined as a 5% decrease in the clinical target volume (CTV) receiving 95% of the prescription dose (V95) or a 5% increase in the dose-volume parameter used as an organ-atrisk constraint. Results: Seventy-three pediatric patients were eligible, with 303 MRI scans (73 simulation and 230 MRI tx scans) available for analysis. The median MRI tx scans per patient was 3 (range, 1-7). Twenty patients (27%) showed anatomic change, with 11 (55%) demonstrating a significant change in delivered plan quality. Significant changes were noted on MRI tx from week 2 (n Z 3) or week 3 (n Z 8). Seven of these 11 patients

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