A method for photon beam Monte Carlo multileaf collimator particle transport

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Dosimetry

Background:

• Monte Carlo (MC) algorithms are the gold standard for accurate patient dose assessment in radiation therapy.
• Accurate dose calculation for intensity-modulated radiation therapy (IMRT) with dynamic multileaf collimators (DMLCs) presents challenges due to complex MLC geometry.
• Full MC simulations of MLC transport are computationally intensive, limiting their routine use.

Purpose of the Study:

• To develop a simplified MC model for multileaf collimators (MLCs) specifically for photon beam IMRT dose computations.
• To reduce the computational time of MC simulations while maintaining accuracy in dose assessment for IMRT.
• To create a versatile MLC model applicable to both dynamic and segmental IMRT delivery.

Main Methods:

• Developed an MC MLC model by separating complex MLC geometry into simpler regions for simplified radiation transport.
• Considered only photon attenuation and first Compton scatter interactions within the MLC, ignoring pair production and electron interactions.
• Validated the model against measurements and full MC simulations for static and dynamic IMRT test cases at 6 MV and 18 MV photon beams.

Main Results:

• The MLC model accurately predicts MLC leakage radiation within 0.1% of open-field dose.
• Entrance dose and beam hardening behind a closed MLC are predicted within +/- 1% or 1 mm.
• Leaf-edge tongue-and-groove effects are predicted within +/- 1% or 1 mm for 95% of points at 6 MV and 88% at 18 MV.

Conclusions:

• The developed MC MLC model provides accurate and efficient dose calculations for IMRT.
• The model is suitable for patient dose calculations, pre-treatment verification, and portal dose dosimetry.
• This advancement facilitates the routine use of accurate MC-based dose calculations in clinical IMRT settings.