Comparisons between MCNP, EGS4 and experiment for clinical electron beams

Understanding the limitations of Monte Carlo codes is essential in order to avoid systematic errors in simulations, and to suggest further improvement of the codes. MCNP and EGS4, Monte Carlo codes commonly used in medical ph ysics, were compared and evaluated against electron depth dose data and exp erimental backscatter results obtained using clinical radiotherapy beams. D ifferent physical models and algorithms used in the codes give significantl y different depth dose curves and electron backscattering factors. The defa ult version of MCNP calculates electron depth dose curves which are too pen etrating. The MCNP results agree better with experiment if the ITS-style en ergy-indexing algorithm is used. EGS4 underpredicts electron backscattering for high-Z materials. The results slightly improve if optimal PRESTA-I par ameters are used. MCNP simulates backscattering well even for high-Z materi als. To conclude the comparison, a timing study was performed. EGS4 is gene rally faster than MCNP and use of a large number of scoring voxels dramatic ally slows down the MCNP calculation. However, use of a large number of geo metry voxels in MCNP only slightly affects the speed of the calculation.