ELPHA: Dynamically deformable liver phantom for real-time motion-adaptive radiotherapy treatments

Area of Science:

• Medical physics
• Radiotherapy technology
• Medical imaging

Background:

• Real-time motion-adaptive radiotherapy requires accurate tracking of intrahepatic tumors.
• Deformable phantoms are essential for developing and validating dynamic treatment strategies.
• Existing phantoms may not fully replicate the complex motion and deformation of the liver.

Purpose of the Study:

• To present a novel deformable liver phantom, ELPHA, for investigating motion detection and mitigation techniques in radiotherapy.
• To assess the phantom's capability for reproducible anatomical deformation and multimodality imaging.
• To evaluate the phantom's utility for time-resolved dosimetry in dynamic treatment scenarios.

Main Methods:

• The dynamically dEformable Liver PHAntom (ELPHA) was constructed using silicone with cellulose for radiographic and ultrasonic contrast.
• Respiratory motion was simulated using an actuator, and motion was tracked using integrated electromagnetic transponders and ultrasound imaging with template matching.
• Time-resolved dosimetry was performed using plastic scintillation dosimeters (PSDs) and radio-chromic film to assess dose delivery accuracy with and without motion compensation (couch tracking).

Main Results:

• ELPHA demonstrated high motion reproducibility (<0.32 mm RMSE) and accurate ultrasound-based motion detection (<1 mm RMSE compared to EM transponders).
• The time-resolved dosimetry system achieved 1 Hz resolution, revealing dose errors that decreased with couch tracking motion compensation.
• Couch tracking improved target dose accuracy to within 5% of the static situation, compared to significant underdosage (-41% to -1%) without compensation.

Conclusions:

• ELPHA is a highly reproducible deformable liver phantom suitable for validating motion detection and mitigation techniques in radiotherapy.
• The phantom's multimodality imaging capabilities enable accurate ultrasound-based motion detection.
• ELPHA serves as a valuable tool for assessing the performance of real-time motion-adaptive radiotherapy systems, including couch tracking.