SU‐D‐207A‐06: Pediatric Abdominal Organ Motion Quantified Via a Novel 4D MRI Method

Abstract

Purpose: To develop a 4D MRI method for assessing respiration‐induced abdominal organ motion in children receiving radiation therapy. Methods: A 4D MRI using internal image‐based respiratory surrogate has been developed and implemented on a clinical scanner (1.5T Siemens Avanto). Ten patients (younger group: N=6, 2–5 years, anesthetized; older group: N=4, 11–15 years) with neuroblastoma, Wilm's tumor rhabdomyosarcoma, or desmoplastic small round cell tumor received free breathing 4D MRI scans for treatment planning. Coronal image slices of the entire abdomen were retrospectively constructed in 10 respiratory phases. A B‐spline deformable registration (Metz et al. 2011) was performed on 4D datasets to automatically derive motion trajectories of selected anatomical landmarks, including the dome and the center of the liver, and the superior edges of kidneys and spleen. The extents of the motion in three dimensions (anteroposterior, AP; mediolateral, ML; superoinferior, SI) and the correlations between organ motion trajectories were quantified. Results: The 4D MRI scans were successfully performed in <20 minutes for all patients without the use of any external device. Organ motion extents were larger in adolescents (kidneys: 3–13 mm SI, liver and spleen: 6–18 mm SI) than in younger children (kidneys:<3mm in all directions; liver and spleen: 1–8 mm SI, 1–5 mm ML and AP). The magnitude of respiratory motion in some adolescents may warrant special motion management. Motion trajectories were not synchronized across selected anatomical landmarks, particularly in the ML and AP directions, indicating inter‐ and intra‐organ variations of the respiratory‐induced motion. Conclusion: The developed 4D MRI acquisition and motion analysis methods provide a non‐ionizing, non‐invasive approach to automatically measure the organ motion trajectory in the pediatric abdomen. It is useful for defining ITV and PRV, monitoring changes in target motion patterns during the treatment course, and studying interplay effects in proton scanning.