[Gate-users] doseActor showing strange behaviour in certain cases

[Hermann Fuchs]

Dear colleagues

It seems that the doseActor exhibits strange unexpected behavior in
certain situations.

In order to clearly define the situation, I am using the standard
Radiation therapy benchmark included with Gate v6.1.
Starting the proton.mac file (the proton simulation part of the
benchmark) in Gate, returns the energy deposited by penetration depth in
the file Config-proton-Edep.txt.

Using the unchanged benchmark file, the expected curve can be observed.

However, if you change the line 
/gate/actor/doseDistribution/stepHitType           random
to
/gate/actor/doseDistribution/stepHitType           middle
the resulting curve changes drastically.
At the left side of the curve, certain bins register an energy deposited
which is by a factor of 20 higher than the expected number, while all
other bins show slightly lower numbers than expected.

This effect is even easier to see if you increase the resolution of the
doseActor by changing the line
/gate/actor/doseDistribution/setResolution         1 1 400 
to
/gate/actor/doseDistribution/setResolution         1 1 1000 

I am unable to explain this effect. As far as I understood, the
stepHitType should only change the position within the bin where the
energy is measured.
This effect can be observed when simulating a few (~500) and as well a
bigger number (~1000000) of particles, so it seems that it is not an
effect caused by to few simulated particles.

Is it possible that there is some unexpected interference caused by
mismatched calculation steps?
How could I avoid this effect?
Simple using random without understanding the underlying effect doesn't
seem like a good idea to me.

I would greatly appreciate your help.

Yours,
Hermann Fuchs

hermann.fuchs at student.tuwien.ac.at