MRI-Compatible Tissue-Equivalent Lung Phantom with Embedded Moving Lesions for Dosimetric Verification
Abstract
Purpose
To develop an MRI-compatible, tissue-equivalent lung phantom with two embedded lesions capable of reproducing realistic breathing motion, including lesion displacement and tissue compression during the breathing cycle. This phantom enables advanced radiotherapy (RT) testing and quality assurance (QA), including MR-guided RT (MRgRT) workflows.
Methods
An anthropomorphic 20 cm triangular-prism lung phantom was constructed using a compressible lung-mimicking composite with a micron-scale porous structure, obtained by combining Polydimethylsiloxane (PDMS) sponge with an agarose hydrogel-in-PDMS emulsion. A 3D-printed scaffold enables axial compression via a movable side that houses two spherical lesions (10 and 15 mm diameter) placed in the lung. Phantom and lesions were split into two halves to accommodate radiochromic film, which could be fixed either to the moving lesions or to the external scaffold. A reproducible motion was generated by an MRI-safe two-port pneumatic cylinder acting on the scaffold’s movable side, controlled by a programmable pneumatic system fed by the room’s pressurized air supply. Lung equivalence was assessed through CT# evaluation. Lesions were filled with a contrast agent to enhance MRI visibility and lesion tracking on a 0.35 T MRI-LINAC. Dosimetric evaluation was performed delivering an MRgRT single fraction of 12.5 Gy to the lesions, both tracking them individually and together, with the film moving synchronously with the target.
Results
The phantom exhibited a CT# of −750 HU, consistent with human lung parenchyma (−650 to −850 HU). Radiochromic film measurements showed a 3% difference of mean dose in the lesions between still and moving phantom, enabling reliable reconstruction of delivered dose distributions with film fixed to the moving lesions.
Conclusion
We developed an MRI-safe dynamic lung phantom that reproduces breathing-induced lesion motion and tissue compression, enabling imaging and dosimetric evaluation of the effective delivered dose to lesions, testing gating and tracking techniques used in MRgRT.