Characterizing the motion management system accuracy on a 1.5T MR-Linac

Area of Science:

• Medical Physics
• Radiation Oncology
• Image-Guided Therapy

Background:

• Advanced motion management techniques reduce treatment volume, enabling dose escalation.
• MRI-Linac systems offer superior contrast and cine imaging for surrogate-less gating.
• Commissioning and performance characterization of such systems are crucial for clinical implementation.

Purpose of the Study:

• To commission a motion management system with gating capabilities on a 1.5T MR-Linac.
• To characterize the system's performance, including stability, tracking accuracy, and latency.
• To evaluate the dosimetric accuracy of clinical treatment plans delivered with the system.

Main Methods:

• Assessed linear accelerator stability across various duty cycles.
• Characterized motion tracking accuracy and gating latency using a motion phantom with varying parameters.
• Created and evaluated clinical treatment plans for multiple tumor sites with different gating strategies.
• Performed end-to-end testing to quantify delivery accuracy for free-breathing and exception gating.

Main Results:

• Beam characteristics remained stable within 0.6% across duty cycles.
• Tracking accuracy was dependent on breathing rate; 1 cm targets posed challenges due to through-plane motion.
• Clinical plans showed average dose differences within 1.8% and >75% gamma passing rates (3%/2 mm).
• Exhale gating strategies exhibited systematic dose shifts due to interplay effects.
• End-to-end testing demonstrated <1-2 mm localization accuracy for moving targets.

Conclusions:

• The Elekta Unity's motion management system offers advanced capabilities for treating moving targets with acceptable uncertainties.
• The system supports accurate radiotherapy delivery, but further research into the clinical impact of specific gating strategies, like exhale gating, is warranted.