当前位置： SCI文献检索 > MEDICAL PHYSICS期刊下所有文献 > On calculating kerma, collision kerma and radiative yields.

On calculating kerma, collision kerma and radiative yields.

Abstract：

PURPOSE:To study the relationships between dose (D), kerma (K), and collision kerma ( K col ) 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). METHODS:Changes 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. RESULTS:The 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 371 (1984) 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 K col vs depth in megavoltage photon beams show that some "conventional wisdom" does not hold in general (e.g., D is not always greater than K past D max ) and in general at 10 cm depth it is found that D≈K and D > K col . The beam radius at which D / K col 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 dosimetry2 (Palmans et al, Med Phys. 2018;45:e1123-e1145). CONCLUSIONS:The changes to EGSnrc make kerma calculations more accurate but previous calculations with electron impact ionization turned off gave close to 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.

journal_name

Med Phys

journal_title

Medical physics

authors

Rogers DWO,Townson RW

doi

10.1002/mp.13744

subject

Has Abstract

pub_date

2019-11-01 00:00:00

pages

5173-5184

issue

eissn

0094-2405

issn

2473-4209

journal_volume

pub_type

杂志文章

在线工具

On calculating kerma, collision kerma and radiative yields.