Optical imaging method to quantify spatial dose variation due to the electron return effect in an MR-linac

Area of Science:

• Medical Physics
• Radiation Oncology
• Imaging Science

Background:

• Accurate dose calculation in MR-guided linear accelerators (MR-linacs) requires Monte Carlo simulations accounting for magnetic field effects.
• Validating these simulations, particularly at tissue-air interfaces where the electron return effect (ERE) is prominent, is challenging due to detector limitations in magnetic fields.

Purpose of the Study:

• To introduce and validate a novel optical imaging technique using Cherenkov and scintillation light to visualize and quantify the electron return effect (ERE) in MR-linac treatment planning.
• To compare experimental ERE data with dose distributions predicted by treatment planning systems (TPS).

Main Methods:

• Utilized an intensified CMOS camera to image two phantoms with designed ERE cavities under irradiation from a 6 MV flattening filter-free (FFF) beam on an MR-linac (0.34 T) and a conventional linac (control).
• Employed optical Cherenkov and scintillation imaging, capturing real-time dose deposition.
• Processed images using Matlab and compared results with TPS dose volumes.

Main Results:

• Optical imaging data validated against control measurements without a magnetic field up to 6 cm depth.
• In the presence of the magnetic field, optical data revealed significant deviations (over 20% in some areas for a 3 cm air cavity) in dose intensity and localization compared to commissioned TPS dose.
• Phantom experiments showed good agreement between optical and film dosimetry in regions unaffected by ERE.

Conclusions:

• Appreciable differences exist between novel experimental dose data and models from current MR-IGRT TPS.
• The novel optical imaging method provides a viable approach for validating dose calculations and improving the accuracy of ERE modeling in MR-linac therapy.