Methodical Differences between the EGSnrc Code and the EGS5 Code

Introduction: A recent work in our group has shown an agreement between calculation results for EGS5 and EGSnrc codes for linac modeling, however, a large run-time difference was found for the same conditions between the two codes. For example, the EGS5 code simulation duration was three times longer than the EGSnrc code for the electron beam of ELEKTA linac. The EGS5 code took a longer period to obtain the same results compared to the EGSnrc code.

Methods: EGS5 Monte Carlo code is a general purpose code for calculating photons and electrons transport for complex geometries in a wide range of energies. EGSnrc Monte Carlo code (with BEAMnrc) was specially developed for medical physics usage, in particular for linac modeling and dose calculations. Both EGS5 an EGSnrc were based on the EGS4 code, for each of these codes changes were made in the electron transport methods, and in the geometrical utilities.

Our study was planned to point out the reason for the runtime difference between the two codes. Time dependency due to the transport parameters, and due to the materials and geometrical definitions were investigated in this research by running the same problem on both codes on the same computer. The simulation geometry was consisting of a small ion-chamber with a radius of 1mm that includes a high Z metal, as an electrode inside a cylindrical water phantom (10 cm X 5 cm).

Results and conclusions: Set of variety simulations were performed using both codes, for several electron energies. We found strong time influence by the ESTEPE parameter values setup, which is the maximum fractional energy loss per step. While setting a larger fractional energy losses per step, reduced simulation run- time was achieved, but this could lead to dose resolution deficiency.

Prior knowledge of the geometry and martial of the simulated problem has to be taken into account for choosing the parameters in order to run it with the EGSnrc code. Hence, for optimal dose, one should define the optimal ESTEPE and step size to achieve the desired dose results resolution. The use of the EGS5 code, based on the electron transport method improvements, is automatically adapted to the desired dose results quality without any user interference. Choosing the proper ESTEPE parameter for a given simulation for the use of EGSnrc is resulting a similar run-time duration as with the use of EGS5.