Determination of the initial beam parameters in Monte Carlo linac simulation

Area of Science:

• Medical Physics
• Radiotherapy
• Computational Dosimetry

Background:

• Accurate determination of initial electron beam phase space parameters is crucial for Monte Carlo simulations and patient dose calculations in radiotherapy.
• Traditional methods rely on time-consuming trial-and-error to match calculated and measured dose distributions, requiring significant expertise and computational power.

Purpose of the Study:

• To propose an easy, efficient, and accurate method for determining initial electron beam parameters for Monte Carlo treatment planning systems (MC-TPS).
• To establish a data library approach for different linac types, simplifying linac modeling for MC-TPS users.

Main Methods:

• Developed a method based on the hypothesis that linacs belong to a limited number of types with consistent head geometry.
• MC-TPS vendors simulate treatment heads and store phase space and dose distribution data for various beam energies and radii.
• Users compare measured dose data with stored calculated distributions to find optimal initial beam energy and radius.

Main Results:

• Evaluated the method on a Varian 21EX linac using EGSNRC/BEAM and EGSNRC/DOSXYZ codes.
• Tested various cost functions for dose distribution comparison, identifying limitations of simple metrics like lateral profile slope.
• Found that statistical uncertainty necessitates accepting a range of energy/radius combinations and requires a comprehensive data set including multiple lateral profiles and central axis depth dose curves.

Conclusions:

• The proposed data library method offers an efficient and accurate alternative to traditional trial-and-error for determining initial beam parameters.
• Careful selection of cost functions and understanding the impact of statistical uncertainty are critical for successful implementation.
• A minimum data set including central axis percent depth dose and several lateral profiles at various depths is necessary for accurate linac modeling.