Magnetic field induced dose effects in radiation therapy using MR-linacs

Area of Science:

• Medical Physics
• Radiation Oncology
• Magnetic Resonance Imaging

Background:

• MR-guided radiotherapy (MRgRT) integrates magnetic resonance imaging (MRI) with linear accelerators (linacs) for real-time visualization during treatment.
• Hybrid MR-linac systems offer enhanced precision by adapting to anatomical and physiological changes.
• However, the presence of a magnetic field introduces unique dosimetric challenges not encountered in conventional radiotherapy.

Purpose of the Study:

• To provide an in-depth review of magnetic field-induced dose effects in 0.35 T and 1.5 T MR-linacs.
• To elucidate the impact of these effects on radiation dose deposition and treatment quality.
• To summarize clinical observations and offer recommendations for mitigating these dosimetric alterations.

Main Methods:

• Review of existing literature on MR-linac systems and their dosimetric properties.
• Analysis of magnetic field interactions with radiation beams and secondary electrons.
• Examination of clinical site-specific treatment data influenced by magnetic fields.

Main Results:

• Magnetic fields alter both in-field and out-of-field doses due to interactions with secondary electrons.
• Key effects include the electron return effect (ERE) and electron stream effect (ESE), which are unique to MR-linac technology.
• ERE and ESE cause dose variations, particularly at tissue interfaces, air cavities, and in specific anatomical regions like the head and neck, breast, lung, and abdomen/pelvis.

Conclusions:

• The interplay between magnetic fields and secondary electrons significantly impacts dose distribution in MRgRT.
• Understanding and mitigating ERE and ESE are essential for optimizing treatment accuracy and clinical outcomes.
• Site-specific strategies are necessary to manage magnetic field-induced dose effects in various cancer treatments.