[Gate-users] Radiationtherapy - Strange Multiple Scattering

[Hermann Fuchs]

Hello

I am trying to simulate simple gaussian shaped helium ion beams in a
homogeneous water phantom. The setup is more or less the same as in the
proton therapy benchmark.
Analyzing the simulated data I got strange results regarding the beams
lateral shape.
One would expect a continuous beam broadening due to Multiple
Scattering.

I have found that using a point beam (e.g from a point source) the beam
broadening looks ok (e.g. according to MS theory).

However, using a gaussian shaped beam the broadening of the beam rises
linearly, decreases again until at the point of the bragg peak the
correct (as expected from MS theory) beam broadening is reached.

I can not explain this. Using 10^7 particles it should not be a
statistics issue.
To illustrate I've attached pictures. I used the recommended Carbon
Physics list, the recommended proton physics list and the
standardhadrontherapy physics list. The effect remains the same although
the pronunciation is slightly different.

Point beam code:
/gate/source/mybeam/gps/particle alpha
/gate/source/mybeam/gps/pos/type Point
/gate/source/mybeam/gps/pos/rot1 0 1 0
/gate/source/mybeam/gps/pos/rot2 1 0 0
/gate/source/mybeam/gps/centre 0. 0. 0.0 cm

Gaussian shaped code:
/gate/source/mybeam/gps/particle alpha
/gate/source/mybeam/gps/pos/type Beam
/gate/source/mybeam/gps/pos/rot1 0 1 0
/gate/source/mybeam/gps/pos/rot2 1 0 0
/gate/source/mybeam/gps/pos/shape Circle
/gate/source/mybeam/gps/pos/centre 0 0 0 cm
/gate/source/mybeam/gps/pos/sigma_x 10 mm
/gate/source/mybeam/gps/pos/sigma_y 10 mm
/gate/source/mybeam/gps/direction 0 0 1

Can anybody explain this effect?

Cheers,
Hermann
-- 
-------------
DI Hermann Fuchs
Div. Medical Radiation Physics
Department of Radiotherapy
Med. Univ. Vienna / AKH Vienna
Währinger Gürtel 18-20
A-1090 Wien

Tel.  + 43 / 1 / 40 400 7271
Mail. hermann.fuchs at meduniwien.ac.at
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