A virtual phantom for patient-specific QA On A 1.5T MR-linac

Area of Science:

• Medical Physics
• Radiotherapy Quality Assurance
• Magnetic Resonance Imaging (MRI) Guided Radiotherapy

Background:

• Quality assurance (QA) for MR-guided radiotherapy, specifically with the ArcCHECK-MR phantom on a 1.5T MR-linac, presents unique challenges due to varying densities.
• Accurate modeling of phantom composition within the treatment planning system (TPS) is crucial for reliable QA measurements.

Purpose of the Study:

• To develop a virtual ArcCHECK-MR phantom within a TPS, accurately representing its distinct density regions for a 1.5T MR-linac.
• To streamline the utilization of the ArcCHECK-MR phantom for quality assurance in MR-guided radiotherapy.
• To validate the virtual phantom's performance using established QA metrics.

Main Methods:

• A 5-layer virtual phantom was constructed in the TPS, modeling the ArcCHECK-MR's composition (Outer, Complex, Detectors, Inner, Insert) using geometric data and dummy CT images.
• Relative electron density (RED) values for different phantom components were determined through ion chamber measurements and analysis of local gamma index passing rates.
• The virtual phantom's performance was evaluated using Unity 7 FFF beams under a 1.5T magnetic field.

Main Results:

• The virtual phantom successfully replicated the ArcCHECK-MR's structure with defined radii and RED values for each layer (PMMA: 1.130, Detectors: 1.000, Complex: 1.200, Setup QA: 1.350).
• Validation showed improved capability compared to bulk overridden phantoms, resulting in an approximate 5-6% increase in local gamma index passing rates (2% dose difference, 2 mm distance-to-agreement).
• Consistent favorable local gamma index passing rates were observed across different 1.5T MR-linac and ArcCHECK-MR unit combinations.

Conclusions:

• A 5-layer virtual ArcCHECK-MR phantom has been successfully generated in a TPS, effectively addressing challenges in 1.5T MR-linac environments.
• This virtual modeling approach enhances QA accuracy and consistency, potentially minimizing uncertainties in phantom creation across institutions.
• The developed virtual phantom offers improved performance and facilitates more reliable quality assurance for MR-guided radiotherapy.