EGS4 was released in 1985 as the new version of EGS3, which was released in 1978. The medical radiotherapy and high energy physics communities both needed an update to the EGS3 code that would track electrons and photons at lower energies. The new release offers tracking thresholds of 10 keV for electrons and 1 keV for photons. [2]
Changes needed to made in the base code in order to track particles to the lower energies. The material data preprocessor (PEGS4) added the flag IAPRIM[3], which is used to align low energy cross sections with experimental data. The transport loop in EGS4 was also altered to fix a infinite loop problem at low energies.
The most important change was to the step size selection algorithm. The user can call a subroutine FIXTMX[4] with a fraction, and the step size selection algorithm will take the maximum step size to be where the particle loses that fraction of its energy. For Pseudo 2 simulations, we used 0.01 as the maximum energy loss fraction.