Monte Carlo simulation of a cobalt-60 beam

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Physics

Background:

• Accurate photon spectra are crucial for radiotherapy dose calculations.
• Cobalt-60 units are widely used in radiation therapy, necessitating precise modeling.
• Previous models often simplified the complex geometry of the source and collimator.

Purpose of the Study:

• To compute detailed photon spectra from an AECL Theratron 780 cobalt-60 unit using Monte Carlo methods.
• To investigate the cause of increased machine output with larger field sizes.
• To compare tissue-air ratio calculations using simulated spectra versus a monochromatic energy approximation.

Main Methods:

• Utilized the Stanford Electron Gamma Shower (EGS) Monte Carlo code for photon spectrum computation.
• Meticulously modeled the geometry and materials of the cobalt-60 source capsule, housing, and collimator.
• Employed a pencil beam model with generated spectra and a monochromatic energy model for tissue-air ratio calculations.

Main Results:

• The study identified scattered photons from the primary definer and adjustable collimator as the cause for increased output with larger field sizes.
• Generated photon spectra were used to calculate tissue-air ratios in water.
• Calculations using simulated spectra showed differences compared to the monochromatic energy model (1.25 MeV).

Conclusions:

• The detailed Monte Carlo simulation accurately models cobalt-60 photon spectra and machine output variations.
• Scattered photons significantly influence the dose distribution and machine output, particularly with increasing field sizes.
• Using simulated spectra in dose calculation models provides a more accurate representation than monochromatic energy approximations for cobalt-60 units.