On calculating kerma, collision kerma and radiative yields

Area of Science:

• Medical Physics
• Radiation Dosimetry
• Computational Physics

Background:

• Accurate dose and kerma calculations are crucial for radiation therapy and diagnostic imaging.
• Existing simulation packages like EGSnrc require refinement for precise energy deposition studies.

Purpose of the Study:

• To investigate the relationships between dose (D), kerma (K), and collision kerma ($$) in photon beams.
• To accurately calculate total radiative yields for electrons and positrons as a function of energy.
• To modify the EGSnrc package for improved collision kerma calculations.

Main Methods:

• Enhanced the EGSnrc package (DOSRZnrc, g applications) to differentiate initiating particle events, particularly electron impact ionization.
• Developed a new application, DOSRZnrcKcol, for direct collision kerma calculation.
• Validated code robustness against simulation parameter changes.

Main Results:

• Modified EGSnrc codes demonstrate increased robustness against simulation parameter variations.
• Calculated radiative yields for electrons differ from ICRU Report 37, with Monte Carlo values being higher due to energy-loss straggling.
• Depth-dependent plots of D, K, and $$ reveal that conventional assumptions may not always hold, with D≈K and $$ at 10 cm depth.

Conclusions:

• EGSnrc modifications enhance kerma calculation accuracy, though prior approximations were often justified.
• DOSRZnrcKcol provides efficient collision kerma calculations, bypassing past approximations.
• Energy-loss straggling, fluorescence, and annihilation in flight increase bremsstrahlung radiation yield.