Monte Carlo implementation, validation, and characterization of a 120 leaf MLC.

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Dosimetry

Background:

• The Varian High Definition 120 leaf multileaf collimator (HD120 MLC) offers improved resolution for radiation therapy fields.
• Accurate characterization of new MLCs is crucial for reliable dose calculations in treatment planning.

Purpose of the Study:

• To investigate the physical characteristics of the HD120 MLC using Monte Carlo (MC) simulations.
• To validate the implementation of the HD120 MLC within the Swiss MC Plan (SMCP) software.

Main Methods:

• Implemented the HD120 MLC in SMCP, adjusting parameters to match transmission measurements for 6 and 15 MV beams.
• Validated the implementation by comparing simulated and measured penumbra, and dose distributions for an IMRT case.
• Generated phase space files below the HD120 MLC for 6 and 15 MV beams to analyze energy spectra, fluence, and energy fluence, comparing with Varian 80 leaf (MLC80) and Millennium M120 (M120 MLC) models.

Main Results:

• MC simulations showed excellent agreement with ionization chamber and film measurements for transmission and penumbra.
• Simulated and measured dose distributions for an IMRT case were in good agreement.
• While energy spectra were similar, mean energy, fluence, and energy fluence differed significantly between HD120 MLC, MLC80, and M120 MLC, with HD120 MLC showing decreased fluence below outer leaves and potential >20% dose reduction due to tongue and groove design.

Conclusions:

• The HD120 MLC was successfully implemented and validated in the SMCP using MC methods.
• The validated SMCP can now accurately calculate dose distributions for treatment plans utilizing the HD120 MLC.