Beam characterisation of the 1.5 T MRI-linac

Area of Science:

• Medical Physics
• Radiation Oncology
• Medical Imaging

Background:

• The integration of magnetic resonance imaging (MRI) with linear accelerators (linacs) offers advanced capabilities for radiation therapy.
• Comprehensive characterization of the radiation beam is a prerequisite for safe and effective clinical implementation of MRI-linacs.

Purpose of the Study:

• To perform a thorough characterization of the Elekta 1.5 T MRI-linac 7 MV FFF radiation beam.
• To verify machine performance and ensure compliance with clinical beam requirements.
• To provide essential data for treatment planning systems.

Main Methods:

• Beam characterization using Semiflex 3D, microDiamond, and Farmer-type detectors in water phantoms.
• Acquisition of data for various field sizes (1x1 cm² to 57x22 cm²).
• Development of new techniques to measure percentage depth dose (PDD) curves, including the electron return effect, and analysis of crossline profiles, penumbral width, and cryostat scatter.

Main Results:

• The Elekta 1.5 T MRI-linac 7 MV FFF beam was characterized for the first time, meeting IEC60976 requirements.
• Electron return effect exhibited a practical range of 1.2 ± 0.1 cm.
• Lorentz force caused crossline profile asymmetry (average shift +0.24 cm), and cryostat scatter contributed 1% dose at the isocenter for a 10x10 cm² field.

Conclusions:

• The characterized 1.5 T MRI-linac 7 MV FFF beam is suitable for clinical use.
• The study provided essential data, including cryostat transmission, for treatment planning.
• This characterization was a key step enabling the first clinical treatments delivered in May 2017.