THE EGS5 CODE SYSTEM

Although the transport mechanics algorithm in PRESTA contains good corrections for longitudinal
and 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 vicinity
of boundaries, is difficult to couple to general purpose combinatorial geometry codes, which may
contain quadric and higher-order surfaces[28]. In the case of quadric surfaces, the determination of
the 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, the
new transport mechanics of EGS5 also yields advantages in computational speed compared to most
models using a boundary sensitive approach. Although applications requiring boundary sensitivity
must exist, we have yet to encounter any in practice. This is certainly an area that would benefit
from 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 distribution
Bielajew et al.[29] modified EGS4 to allow for angular distributions employing the Schiff formula
from a review article by Koch and Motz[91]. Standard EGS4 makes the approximation that the
angle of the bremsstrahlung photon with respect to the initiating charged particle’s direction is
Θ = 1/E 0 where E 0 is the initiating charged particle’s energy in units of the electron rest mass
energy. It was acknowledged that this might be a bad approximation for thin-target studies, but
it was expected that there would be no effect in thick-target studies since multiple scattering
would “wash-out” the initial bremsstrahlung angular distribution and that an average value would
be sufficient. However, thick-target studies in the radiotherapy range showed dramatic evidence of
this approximation as a calculation artifact[54]. Angular distributions near the central axis changed
by as much as 40%! Thick-target studies at diagnostic energies also showed the artifact which was
eliminated through use of the new sampling technique[124]. This new capability was carried over
to EGS5.
ICRU37 collision and radiative stopping powers
Duane et al.[52] modified PEGS4 to give collision stopping powers identical to those of ICRU Report
37[20, 79]. The NBS (now NIST) database EPSTAR[148] which was used to create the ICRU tables
was employed. The modifications also allow the user to input easily an arbitrary density-effect
correction. This change is relatively small but crucial if one is doing detailed stopping-power-ratio
studies[101, 137].
In a related work, Rogers et al.[139] adapted PEGS4 to make the radiative stopping powers
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