Development and first implementation of a novel multi‐modality cardiac motion and dosimetry phantom for radiotherapy applications
Authors: Gregg Kenneth W. , Ruff Chase , Koenig Grant , Penev Kalin I. , Shepard Andrew , Kreissler Grace , Amatuzio Margo , Owens Cameron , Nagpal Prashant , Glide‐Hurst Carri K. 
Published: July 2024

Background

Cardiac applications in radiation therapy are rapidly expanding including magnetic resonance guided radiation therapy (MRgRT) for real‐time gating for targeting and avoidance near the heart or treating ventricular tachycardia (VT).

Purpose

This work describes the development and implementation of a novel multi‐modality and magnetic resonance (MR)‐compatible cardiac phantom.

Methods

The patient‐informed 3D model was derived from manual contouring of a contrast‐enhanced Coronary Computed Tomography Angiography scan, exported as a Stereolithography model, then post‐processed to simulate female heart with an average volume. The model was 3D‐printed using Elastic50A to provide MR contrast to water background. Two rigid acrylic modules containing cardiac structures were designed and assembled, retrofitting to an MR‐safe programmable motor to supply cardiac and respiratory motion in superior‐inferior directions. One module contained a cavity for an ion chamber (IC), and the other was equipped with multiple interchangeable cavities for plastic scintillation detectors (PSDs). Images were acquired on a 0.35 T MR‐linac for validation of phantom geometry, motion, and simulated online treatment planning and delivery. Three motion profiles were prescribed: patient‐derived cardiac (sine waveform, 4.3 mm peak‐to‐peak, 60 beats/min), respiratory (cos4 waveform, 30 mm peak‐to‐peak, 12 breaths/min), and a superposition of cardiac (sine waveform, 4 mm peak‐to‐peak, 70 beats/min) and respiratory (cos4 waveform, 24 mm peak‐to‐peak, 12 breaths/min). The amplitude of the motion profiles was evaluated from sagittal cine images at eight frames/s with a resolution of 2.4 mm × 2.4 mm. Gated dosimetry experiments were performed using the two module configurations for calculating dose relative to stationary. A CT‐based VT treatment plan was delivered twice under cone‐beam CT guidance and cumulative stationary doses to multi‐point PSDs were evaluated.

Results

No artifacts were observed on any images acquired during phantom operation. Phantom excursions measured 49.3 ± 25.8%/66.9 ± 14.0%, 97.0 ± 2.2%/96.4 ± 1.7%, and 90.4 ± 4.8%/89.3 ± 3.5% of prescription for cardiac, respiratory, and cardio‐respiratory motion profiles for the 2‐chamber (PSD) and 12‐substructure (IC) phantom modules respectively. In the gated experiments, the cumulative dose was <2% from expected using the IC module. Real‐time dose measured for the PSDs at 10 Hz acquisition rate demonstrated the ability to detect the dosimetric consequences of cardiac, respiratory, and cardio‐respiratory motion when sampling of different locations during a single delivery, and the stability of our phantom dosimetric results over repeated cycles for the high dose and high gradient regions. For the VT delivery, high dose PSD was <1% from expected (5–6 cGy deviation of 5.9 Gy/fraction) and high gradient/low dose regions had deviations <3.6% (6.3 cGy less than expected 1.73 Gy/fraction).

Conclusions

A novel multi‐modality modular heart phantom was designed, constructed, and used for gated radiotherapy experiments on a 0.35 T MR‐linac. Our phantom was capable of mimicking cardiac, cardio‐respiratory, and respiratory motion while performing dosimetric evaluations of gated procedures using IC and PSD configurations. Time‐resolved PSDs with small sensitive volumes appear promising for low‐amplitude/high‐frequency motion and multi‐point data acquisition for advanced dosimetric capabilities. Illustrating VT planning and delivery further expands our phantom to address the unmet needs of cardiac applications in radiotherapy.