MOSFET dosimeter characterization in MR-guided radiation therapy (MRgRT) Linac.

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Magnetic Resonance-guided Radiation Therapy (MRgRT) systems integrate magnetic fields with radiation delivery.
• Accurate dosimetry is crucial for patient safety and treatment efficacy in MRgRT.
• The performance of medical dosimeters within strong magnetic fields requires thorough characterization.

Purpose of the Study:

• To characterize the performance of metal-oxide-semiconductor field-effect transistor (MOSFET) dosimeters in a 0.345 T magnetic field relevant to MRgRT.
• To evaluate the accuracy and reliability of MOSFETs for in-vivo dosimetry during MRgRT procedures.

Main Methods:

• A commercial MOSFET dosimetry system was commissioned on an MRIdian MRgRT system.
• Key dosimetric parameters including calibration, linearity, angular dependence, field size dependence, percentage depth dose (PDD), and output factors were assessed.
• Intensity modulated radiation therapy (IMRT) quality assurance (QA) was performed and compared with ionization chamber measurements.

Main Results:

• MOSFET measurements showed no significant impact from the 0.345 T magnetic field, with variations less than 2% in calibration factors.
• Excellent linearity (R² = 0.999) was observed for a wide range of monitor units.
• MOSFET PDD and output factor measurements were in close agreement with ionization chamber data (within 1% and 2.2%, respectively).
• IMRT QA results demonstrated good agreement between MOSFET and ionization chamber measurements across different treatment sites.

Conclusions:

• MOSFET dosimeters are suitable for use in 0.345 T MRgRT systems without magnetic field interference.
• Their performance characteristics are comparable to established dosimetry methods like ionization chambers and thermoluminescent detectors (TLDs).
• MOSFETs offer a viable alternative for real-time in-vivo dosimetry in MRgRT applications.