Technical Note: End-to-end verification of an MR-Linac using a dynamic motion phantom

Area of Science:

• Medical Physics
• Radiation Oncology
• Medical Imaging

Background:

• Magnetic Resonance-Linear Accelerator (MR-Linac) systems integrate MRI and linear accelerators for adaptive radiation therapy.
• Real-time imaging and superior tissue contrast enable precise targeting and potential reduction of planning target volume (PTV) margins.

Purpose of the Study:

• To verify the dosimetric accuracy of an MR-Linac system when treating moving targets.
• To assess treatment errors associated with different motion patterns and adaptive strategies.

Main Methods:

• An end-to-end test was conducted on the Elekta Unity MR-Linac using a 4D Dynamic Thorax Phantom.
• Dose measurements were performed in various phantom regions using an ion chamber across seven motion patterns.
• Adaptation workflows (Adapt-to-Position and Adapt-to-Shape) and dose distribution accuracy were evaluated using a diode array phantom.

Main Results:

• All treatment plans achieved excellent quality assurance (QA) results, with over 95% passing rates for gamma analysis.
• Adaptive planning on the MR-Linac yielded high-quality plans.
• Measured doses within the target volume closely matched calculated doses.

Conclusions:

• The investigated MR-Linac system demonstrated expected performance for adaptive treatment of moving targets.
• Accurate 3D MR visualization and contouring facilitated online adaptive planning.
• Clinical differences between measured and calculated doses were acceptable across diverse motion patterns.