Dose perturbations at tissue interfaces during parallel linac-MR treatments: The "Lateral Scatter Electron Return...

Area of Science:

• Medical Physics
• Radiation Oncology
• Medical Imaging

Background:

• Medical linear accelerators (linacs) are integrated with magnetic resonance (MR) imaging for radiation therapy, creating parallel (LMR-B∥) and perpendicular (LMR-B⊥) systems.
• The MR's magnetic field can perturb radiation dose delivery due to effects like the electron return effect (ERE) and electron streaming effects (ESEs) in LMR-B⊥ systems.
• A dose collimating effect has been observed in LMR-B∥ systems, but specific asymmetric dose perturbations at material interfaces were not well-characterized.

Purpose of the Study:

• To report on a novel asymmetric dose perturbation, termed Lateral Scattered Electron Return Effect (LS-ERE), occurring at material interfaces in parallel linac-MR (LMR-B∥) systems.
• To quantify the LS-ERE and its dependence on magnetic field strength, field size, and material interfaces.
• To investigate methods for LS-ERE cancellation and compare it with other magnetic field-induced dose perturbations.

Main Methods:

• Utilized BEAMnrc and EGSnrc Monte Carlo (MC) codes to simulate a 6 FFF beam from a 0.5-T linac-MR at 0.5 T and 1.5 T.
• Simulated various phantom material-interface combinations and field sizes, including modulated fields, to quantify LS-ERE in patient-specific CT datasets (head, breast, lung).
• Validated MC simulations with GafChromic film measurements at a water-air interface in a 0.5-T LMR-B∥ system and simulated ERE for LMR-B⊥ systems for comparison.

Main Results:

• LS-ERE was found to be largely independent of field size for fields > 1x1 cm², with asymmetries up to ±9.0% at tissue-air interfaces and ±4.1% at tissue-lung interfaces at 0.5T/1.5T.
• LS-ERE magnitude and extent decreased with increasing magnetic field strength and increased with density gradients.
• Techniques like opposing (POP) fields and magnetic field reversal effectively reduced LS-ERE asymmetries. LS-ERE contributed to approximately 30% of the observed increase in skin dose.

Conclusions:

• Lateral Scattered Electron Return Effect (LS-ERE) is a significant dose perturbation present at material interfaces in parallel linac-MR (LMR-B∥) systems.
• Understanding and accounting for LS-ERE is critical for accurate treatment planning system (TPS) evaluation in LMR-B∥ treatments, particularly in regions with steep tissue density gradients.
• The findings highlight the importance of considering these specific dose perturbations for safe and effective MR-guided radiation therapy.