On calculating kerma, collision kerma and radiative yields.

Abstract

PURPOSE\nTo study the relationships between dose (D), kerma (K) and collision kerma (Kcol) in photon beams and to investigate total radiative yields for electrons and positrons as a function of energy. To do this accurately required calculating collision kerma directly as a function of position in a phantom and making changes to the EGSnrc package (including DOSRZnrc and g applications).\n\n\nMETHODS\nChanges were made to the EGSnrc system to allow the user to distinguish events according to their initiating process, most importantly relaxation particles initiated by electron impact ionization as opposed to initiated by photons, especially those events depositing energy below energy cutoffs after relaxation events. Appropriate changes were made to the applications DOSRZnrc and g and a new application, DOSRZnrcKcol, was written.\n\n\nRESULTS\nThe modified codes are much more robust against changes in simulation parameters such as ECUT and AE and whether or not electron impact ionization is included in the simulation. The radiative yields for electrons generally differ from values in ICRU Report 37 which only account for radiative losses due to bremsstrahlung. The Monte Carlo calculated g(brems) values are generally greater than the ICRU 37 values due to energy-loss straggling. Plots of D, K and Kcol vs depth in megavoltage photon beams show that some conventional wisdom doesn t hold in general (e.g., D is not always greater than K past Dmax) and in general at 10 cm depth it is found that D approx= K and D > Kcol. The beam radius at which D/Kcol reaches its saturation value depends strongly on the threshold used to define reaching saturation and is generally greater than the radius for lateral charged particle equilibrium used in the TRS-483 Code of Practice for small beam dosimetry.\n\n\nCONCLUSIONS\nThe changes to EGSnrc make kerma calculations more accurate but previous calculations with electron impact ionization turned off gave close to the correct results. The application DOSRZnrcKcol makes calculating collision kerma more efficient and avoids various approximations used in the past although those approximations are shown to be justified. Including energy-loss straggling when calculating bremsstrahlung radiation yield increases the value. Fluorescence losses and annihilation in flight further increase the radiation yield of electrons and positrons. Results demonstrate the effects of EGSnrc using electron bremsstrahlung production cross sections for positrons and failing to model positron impact ionization.