The EGS (Electron Gamma Shower) computer code system is a general purpose package for the Monte Carlo simulation of the coupled transport of electrons and photons in an arbitrary geometry for particles with energies from a few keV up to several TeV. It was developed at SLAC. Some have referred to EGS as the de facto gold standard for clinical radiation dosimetry. FLURZnrc is an add-on for EGS that calculates particle flux. CAVRZnrc is an add-on for ion chamber simulations. SPRRZnrc calculates stopping power ratios.
The SLAC version of EGS is no longer actively developed. There are two forks which are being actively maintained:
References
EGS (program) Wikipedia(Text) CC BY-SA