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.