Geant4.9.6 vs 10.7.1 for 56.4 keV photons

Dear Geant4 users,

I would really appreciate some help in identifying the differences between how Geant4.9.6 patch 2 and 4.10.7 patch 1 deal with scattering of 56.4 keV photons in soft tissue.

Specifically, I’m running some test cases from the American Association of Physicists in Medicine report 195 (2015) to check that my diagnostic radiology simulations code provides results that are in-line with the AAPM results.

AAPM report 195 is available on-line here: AAPM 195

AAPM report 195 used Geant4.9.6 patch 2 with EM standard option 4; I’m using Geant4.10.7 patch 1 with EM standard option 4.

Part of test case 2 in AAPM report 195 involves simulating:

  • a soft-tissue block
  • a scoring plane behind the block
  • a 56.4 keV photon source collimated to the scoring plane

For this test case the AAPM report includes results for:

  • energy deposited in the soft tissue block as a whole
  • energy deposited in several volumes of interest in the block
  • energy arriving at several regions of interest of the scoring plane, split into primary, total scatter, single Compton, single Rayleigh and multiple scatter

When using Geant4.10.7 patch 1 I find that:

  • 1.4 % less energy is deposited in the tissue block compared with the AAPM results
  • up to 6 % less energy deposited in the tissue block volumes of interest compared with the AAPM results
  • ~ 2 to 4 % more energy arrives at the scoring plane regions from scattered photons compared with the AAPM results

I think that my results suggest that something has changed in the Geant4 scattering physics between Geant4.9.6 patch 2 and 4.10.7 patch 1.

Can someone tell me if the differences between my results and those of the AAPM are expected based on changes in the way that Geant4 calculates scatter between versions 4.9.6 patch 2 and 4.10.7 patch 1, both using EM standard option 4?

Many thanks for any help that you can offer.

Kind regards,

David

There have been changes in the configuration of the Geant4 EM standard option 4
physics list between version 9.6patch-2 and 10.7patch-1.

The most significant one is related to the model used to simulate e-/e+ multiple Coulomb scattering
(below 100 MeV kinetic energy): the semi-empirical Urban model is used in 9.6 while the fully theory based Goudsmit-Saunderson (GS) model is utilised since version 10.4.

This latter also contains several important higher order corrections when used in the EM standard option 4 physics list since Geant4 version 10.4. These, together with its so called “error-free” stepping (see more details here), provide a significantly more accurate EM shower simulation compared to the earlier Urban model.

The cited report contains simulation results obtained by using EGSnrc, Geant4, MCNPX and PENELOPE. We expect, that your results (obtained with Geant4 version 10.7) moved closer to those in the report obtained by EGSnrc (and PENELOPE) compared to the reported Geant4 results (using version 9.6). This is because the above-mentioned accurate GS e-/e+ multiple Coulomb scattering model (as used in the EM standard option 4 physics constructor since version 10.4) is rather similar to that used in EGSnrc (the same theoretical foundation including all the corrections and the stepping algorithm). This increased level of agreement, between the simulation results obtained by using the new Geant4 GS model with its option 4 configuration and those provided by EGSnrc, have already been confirmed by several applications.

Note, that while we expect changes in the simulation results between Geant4 versions 9.6p2 and 10.7p1 due to the above-mentioned improvements (among others, since e.g. the parametrisation of the photoelectric cross sections have also been updated), we cannot tell you for sure that what you see is due to and only due to these as we have not made the corresponding simulations with both versions. However, you might try to run your simulation also with Geant4 version 9.6p2 (available here) that should reproduce the results provided in the cited report.