Feasibility Study of a 4D Respiratory Phantom for Quality Assurance of Particle Therapy

Abstract

Purpose: Respiratory motion in particle therapy necessitates quality assurance (QA) tools capable of decoupling mechanical/temporal performance from complex dosimetric perturbations. This study aims to develop a four-dimensional internal−external respiratory motion phantom (4D IERM Phantom) to simulate patient-specific respiratory patterns and tumor motion for particle therapy QA applications. Methods: The 4D IERM Phantom consists of a control unit and an operating unit. The control unit was constructed using a printed circuit board based on an Arduino Mega 2560, while the operating unit was divided into two main parts: the abdominal and chest regions. The abdominal region mimics 3D target motion in the anterior−posterior (AP), superior−inferior, and left−right directions using three stepper motors, whereas the chest region simulates 1D motion in the AP direction with a single stepper motor. Additionally, the 4D IERM Phantom is designed to accommodate dose measurements using a multi-active volume ion chamber and a MatriXX for both the abdominal and chest regions. For validation, actual patient respiratory data with a 50-ms time interval and a 0.2 mm step size were used. Results: The 4D IERM Phantom was validated using respiratory data from 72 actual patients. The phantom successfully simulated patient-specific respiratory patterns and tumor motion across all four independent axes. Motion fidelity analysis demonstrated high accuracy: the Pearson correlation coefficient (r) exceeded 0.97 across all four axes. Quantitative metrics confirmed high precision, with root mean square error values ranging from 0.1008 mm to 0.7162 mm, and maximum absolute error contained within approximately 2 mm for the AP axes. Conclusions: The 4D IERM Phantom mimics complex, patient-specific respiratory motion and provides a robust, feasible solution for the isolated mechanical and temporal validation of 4D motion management systems in particle therapy.