Adaptive Proton Therapy for Pediatric Patients: Improving the Quality of the Delivered Plan With On-Treatment MRI.

Abstract

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. 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-at-risk constraint. 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 = 3) or week 3 (n = 8). Seven of these 11 patients (64%) had a significantly reduced CTV V95 (median decrease, 7.6%; range, 5%-16%). Four (36%) had a significantly increased dose to the brain stem, hippocampus, and/or optic apparatus. Eight had a suprasellar low-grade glioma or head and neck rhabdomyosarcoma. On-treatment MRI was useful in detecting anatomic changes during proton therapy. MRI-based offline adaptation improved plan quality for most patients with anatomic changes. Further studies should determine the clinical value of MRI-based adaptive therapy for pediatric patients.