[Gate-users] Dose profile difference due to additional geometry

[Jana Kretschmer]

Dear Gate Users,

I am scoring the lateral edep distribution in 2 cm depth in a water phantom resulting from a very thin slit source (0.1 mm x 10 cm) of 150 MeV protons. I performed two simulations: 

Case A: In a first simulation, the only geometry in the water phantom geometry was the scoring volume. The resulting dose profile is shown in the attached figure (red line, FWHM: 0.28 mm)

Case B: In a second simulation, I defined an additional box geometry of 3 x 4 x 1.1 cm3 of water enclosing the scoring volume but kept everything else as it was in case A such that I expected the same result. However the profile was much more narrow as shown in the figure as black line (FWHM: 0.13 mm). 

It looks like the additional geometry strongly influences the scattering of the protons in the water phantom. I tested some more configurations and was able to reproduce profile A using only a few modifications to the input of case B:
Using case B input + cuts of 0.02 mm in the water phantom (FWHM: 0.28 mm)
Using case B input but changing the additional box geometry to 3 x 4 x 3.5 cm3 (FWHM: 0.28 mm)

Does anyone know what leads to the difference in the profiles? I would really appreciate your help. You can find my input files attached. I am using Gate V8.0 with Geant4v10.4.1. 

Best regards
Jana Kretschmer 
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