Calculations for plane-parallel ion chambers in 60Co beams using the EGSnrc Monte Carlo code

Area of Science:

• Medical Physics
• Computational Dosimetry
• Radiation Oncology

Background:

• Accurate dosimetry is crucial for effective radiation therapy.
• Correction factors for ionization chambers are essential for precise dose calculations.
• Previous Monte Carlo simulations had limitations in accuracy and efficiency.

Purpose of the Study:

• To calculate correction factors (Kcomp, Pwall) for plane-parallel ionization chambers in 60Co beams using EGSnrc.
• To improve the robustness and accuracy of Monte Carlo simulations for these correction factors.
• To assess the impact of simulation parameters and cross-section uncertainties on calculated factors.

Main Methods:

• Utilized the EGSnrc Monte Carlo simulation system for calculations.
• Employed a more robust calculation method than previously reported.
• Investigated the effect of particle production thresholds and transport energies.
• Analyzed uncertainties arising from photon and electron cross-section data.

Main Results:

• Identified a minor conceptual error in axial nonuniformity correction (Kan) with a 0.2% effect.
• Validated the assumption of Pwall in-phantom being numerically equal to Kcomp (water buildup cap) with <0.06% accuracy.
• Calculated Kcomp factors have uncertainties of 0.14% (photon) and 0.24% (electron).
• Calculated Kwall factors have 0.03% uncertainty from photon cross-sections and negligible from electron cross-sections.
• Results are systematically higher by 0.8% compared to previous EGS4/PRESTA calculations.

Conclusions:

• The refined EGSnrc simulations provide more accurate correction factors for ionization chambers.
• The improved accuracy leads to better agreement with experimental data.
• These findings contribute to enhanced precision in radiation dosimetry and treatment planning.