High-resolution entry and exit surface dosimetry in a 1.5 T MR-linac

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Transverse magnetic fields in MR-linacs alter electron trajectories, impacting surface and near-surface radiation doses.
• Accurate surface dosimetry is crucial for patient safety and treatment efficacy in advanced radiotherapy techniques.

Purpose of the Study:

• To investigate the performance of the MOSkin™ MOSFET detector for high-resolution surface and near-surface dosimetry on a transverse MR-linac (Elekta Unity).
• To compare MOSkin™ measurements with simulations (Geant4, Monaco TPS) and EBT-3 film dosimetry.

Main Methods:

• Measurements of surface and near-surface percentage depth doses using the MOSkin™ detector across various field sizes.
• Comparison of MOSkin™ data with Geant4 Monte Carlo simulations, Monaco treatment planning system (TPS) calculations, and EBT-3 film measurements.
• Analysis of dose discrepancies at beam entry and exit surfaces, including the electron return effect.

Main Results:

• MOSkin™ measured entry surface doses ranged from 9.9% to 13.4% relative to Dmax across field sizes from 1x1 cm² to 22x22 cm².
• Maximum percent differences between MOSkin™ and other methods were 1.0% (Geant4 entry), 2.8% (TPS entry), 14.3% (film entry), 3.2% (Geant4 exit), 3.7% (TPS exit), and 5.7% (film exit).
• The electron return effect caused dose increases up to 15.4% (MOSkin™) and 17.9% (Geant4) for larger fields.

Conclusions:

• The MOSkin™ detector demonstrates suitability for accurate surface and near-surface dosimetry in transverse MR-linac applications.
• The study validates MOSkin™ for precise assessment of surface doses and the electron return effect in MR-guided radiotherapy.