A dynamic anthropomorphic phantom for end-to-end testing in image- and surface-guided adaptive radiotherapy

Respiratory and digestive motion can compromise the accuracy of radiotherapy (RT), affecting tumor targeting and healthy tissue sparing. Current phantoms often fail to replicate complex organ movements and lack compatibility with advanced imaging modalities like magnetic resonance imaging (MRI) and 4D computed tomography (4DCT), limiting their utility in adaptive radiotherapy. The BRaVIDA (Breathing Radiotherapy Visual monitoring, Imaging, and Dosimetric Anthropomorphic) phantom addresses these limitations by simulating realistic organ motion during breathing and digestion. Its MRI compatibility allows for comprehensive testing in adaptive radiotherapy workflows, improving motion management and treatment precision for both photon and ion beam therapies.

This study introduces the BRaVIDA (Breathing Radiotherapy Visual monitoring, Imaging, and Dosimetric Anthropomorphic) phantom, a novel anthropomorphic phantom designed to simulate respiratory motion and support end-to-end testing in adaptive radiotherapy. The aim is to improve motion management, imaging accuracy, and dosimetric validation for radiotherapy treatments.

The BRaVIDA phantom is designed with realistic anatomical structures of the thorax and abdomen and may be equipped with dosimeters. It simulates respiratory and digestive motion using an in-house developed hydraulic system. The phantom was imaged by CT and MRI, using different protocols (CT, MRI, 4DCT, and 4DMRI) to evaluate image contrast and motion parameters. Dosimetric properties were assessed by measuring the relative electron density (RED) and the stopping power ratio (SPR) of ion beams and the results were compared with the values of the treatment planning system (TPS).

The phantom demonstrated realistic image contrast in CT and MRI, with anthropomorphic characteristics similar to human tissues. Motion amplitudes for various organ models (pancreas, stomach, liver) were successfully measured using 4DCT and 4DMRI. Dosimetric testing showed that the RED and SPR values of BRaVIDA align closely with TPS reference values. Deviations in photon attenuation were below 1.5% for all phantom materials.

BRaVIDA presents a versatile, MRI-compatible phantom exhibiting anthropomorphic image contrast, inter- and intrafractional motion as well as radiation attenuation in photon beams, and equipping the phantom with detectors allows for full end-to-end-tests in adaptive image- and surface-guided photon RT workflows without further adaptions. For ion beams, the SPR values in the TPS have to be adapted to the measured data to assure correct range calculation in the phantom. The phantom presents a valuable and accurate tool for clinical and research applications in adaptive RT.