THE EGS5 CODE SYSTEM

Although the transport mechanics algorithm in PRESTA contains good corrections for longitudinaland lateral displacements (as well as average energy), it has been abandoned in EGS5.This change was motivated primarily by the desire to eliminate PRESTA’s boundary algorithm,which, though theoretically the best way for handling elastic-scattering ambiguities in the vicinityof boundaries, is difficult to couple to general purpose combinatorial geometry codes, which maycontain quadric and higher-order surfaces[28]. In the case of quadric surfaces, the determination ofthe closest distance to a boundary may involve solving for the roots of a sixth-order polynomial.While such algorithms may be devised, specifically designed with this application in mind[132],they are too slow for routine application.Thus, in addition to providing improved treatment of longitudinal and lateral displacement, thenew transport mechanics of EGS5 also yields advantages in computational speed compared to mostmodels using a boundary sensitive approach. Although applications requiring boundary sensitivitymust exist, we have yet to encounter any in practice. This is certainly an area that would benefitfrom more attention, as a boundary sensitive component could be added without much effort.Moreover, the current EGS5 transport mechanics could be adapted easily to most tracking codes,which perform ray tracing in combinatorial geometries without boundary sensitivity.Bremsstrahlung angular distributionBielajew et al.[29] modified EGS4 to allow for angular distributions employing the Schiff formulafrom a review article by Koch and Motz[91]. Standard EGS4 makes the approximation that theangle of the bremsstrahlung photon with respect to the initiating charged particle’s direction isΘ = 1/E 0 where E 0 is the initiating charged particle’s energy in units of the electron rest massenergy. It was acknowledged that this might be a bad approximation for thin-target studies, butit was expected that there would be no effect in thick-target studies since multiple scatteringwould “wash-out” the initial bremsstrahlung angular distribution and that an average value wouldbe sufficient. However, thick-target studies in the radiotherapy range showed dramatic evidence ofthis approximation as a calculation artifact[54]. Angular distributions near the central axis changedby as much as 40%! Thick-target studies at diagnostic energies also showed the artifact which waseliminated through use of the new sampling technique[124]. This new capability was carried overto EGS5.ICRU37 collision and radiative stopping powersDuane et al.[52] modified PEGS4 to give collision stopping powers identical to those of ICRU Report37[20, 79]. The NBS (now NIST) database EPSTAR[148] which was used to create the ICRU tableswas employed. The modifications also allow the user to input easily an arbitrary density-effectcorrection. This change is relatively small but crucial if one is doing detailed stopping-power-ratiostudies[101, 137].In a related work, Rogers et al.[139] adapted PEGS4 to make the radiative stopping powers14