Clinical reference dosimetry for the 0.5 T inline rotating biplanar Linac-MR

Area of Science:

• Medical Physics
• Radiation Oncology
• Imaging Technology

Background:

• The first clinical 0.5 T biplanar linear accelerator-magnetic resonance (Linac-MR) system presents unique dosimetry challenges.
• Standard protocols like TG-51 are difficult to apply due to system specifics such as SAD = 120 cm and a 60 cm x 110 cm bore clearance.

Purpose of the Study:

• To develop and validate a practical clinical reference dosimetry protocol for the 0.5 T biplanar Linac-MR system.
• To ensure accurate dose delivery and patient safety in a novel hybrid imaging and treatment modality.

Main Methods:

• Utilized tissue phantom ratios (TPR) to determine the %dd10x beam quality specifier.
• Employed TG-51 addendum equations for ion chamber correction factors, cross-calibrating the Exradin A12 chamber.
• Assumed negligible magnetic field effects on chamber response for parallel fields.
• Validated the protocol using Monte Carlo (EGSnrc) simulations and external dose audits with various dosimeters.

Main Results:

• Monte Carlo simulations confirmed the protocol-determined kQ,msr values and the assumption of negligible magnetic field effects (kB = 1).
• External dosimetry audits validated the overall accuracy of the developed protocol for the 0.5 T Linac-MR.
• Dosimetric differences when ramping down the magnetic field were minimal (0.1%), within experimental uncertainty.

Conclusions:

• A TPR20,10 approach for beam quality determination is accurate for clinical dosimetry on the 0.5 T biplanar Linac-MR.
• The parallel magnetic field of the 0.5 T system has minimal impact on ion chamber response.
• The proposed protocol provides a reliable method for reference dosimetry in this advanced Linac-MR system.