Setup Uncertainty of Pediatric Brain Tumor Patients Receiving Proton Therapy: A Prospective Study

Simple Summary Proton therapy enables the delivery of a high radiation dose to tumors while sparing surrounding normal tissues. Inaccurate patient positioning may lead to underdosing of the targeted tumor and overdosing of nearby healthy tissues. Before a course of proton therapy, a CT scan is acquired for treatment planning with the patient in the treatment position, and this image dataset is used as a reference for patient localization at each treatment fraction. Radiation therapists perform daily cone-beam CT (CBCT) image guidance to align the patient to closely match the planning CT and minimize inaccuracies in radiation delivery. In this study, CBCT scans were systematically collected with clinical and treatment information to investigate questions related to image guidance, setup uncertainty, and patient motion during the treatment of pediatric brain tumors. Knowledge gained from this study provides a basis for designing safe and optimal proton treatments.Abstract This study quantifies setup uncertainty in brain tumor patients who received image-guided proton therapy. Patients analyzed include 165 children, adolescents, and young adults (median age at radiotherapy: 9 years (range: 10 months to 24 years); 80 anesthetized and 85 awake) enrolled in a single-institution prospective study from 2020 to 2023. Cone-beam computed tomography (CBCT) was performed daily to calculate and correct manual setup errors, once per course after setup correction to measure residual errors, and weekly after treatments to assess intrafractional motion. Orthogonal radiographs were acquired consecutively with CBCT for paired comparisons of 40 patients. Translational and rotational errors were converted from 6 degrees of freedom to a scalar by a statistical approach that considers the distance from the target to the isocenter. The 95th percentile of setup uncertainty was reduced by daily CBCT from 10 mm (manual positioning) to 1-1.5 mm (after correction) and increased to 2 mm by the end of fractional treatment. A larger variation existed between the roll corrections reported by radiographs vs. CBCT than for pitch and yaw, while there was no statistically significant difference in translational variation. A quantile mixed regression model showed that the 95th percentile of intrafractional motion was 0.40 mm lower for anesthetized patients (p=0.0016). Considering additional uncertainty in radiation-imaging isocentricity, the commonly used total plan robustness of 3 mm against positional uncertainty would be appropriate for our study cohort.