[Gate-users] Divergent photon beam of point source and dose profile along the central axis of water phantom

[Loic Grevillot]

Hi Stathis,

You defined an actor of 1x1x20 cm, which is centered in a volume of
50x50x50 cm. Therefore you do not get the first 15 cm and the build-up.
If you do not mentioned the size of the actor, the actor will automatically
get the size of the volume.
Just try:
#/gate/actor/doseDistribution/setSize 1 1 20 cm
/gate/actor/doseDistribution/setResolution 1 1 5000
 instead of
/gate/actor/doseDistribution/setSize 1 1 20 cm
/gate/actor/doseDistribution/setResolution 1 1 2000

If you absolutely want to use a size of 20 cm 1x1x20 cm, you should
translate the actor position:
/gate/actor/doseDistribution/setPosition 0 0 -15 cm
/gate/actor/doseDistribution/setSize 1 1 20 cm
/gate/actor/doseDistribution/setResolution 1 1 2000

Looking forward for the tutorial,

Cheers,
LG

Loïc


2014-02-03 Stathis Kamperis <ekamperi at gmail.com>:

> Dear Gate users,
>
> for once more I will appeal to your expertise. I'd like to simulate a
> point source of photons 20 MeV that diverges in space so that when it
> hits my water tank it forms a 10x10 cm field. Subsequently, I'd like
> to measure the dose depth along the central axis.
>
> I define the water phantom as:
>
> /gate/world/daughters/name                      waterTank
> /gate/world/daughters/insert                      box
> /gate/waterTank/placement/setTranslation 0 0 25 cm
> /gate/waterTank/geometry/setXLength      50 cm
> /gate/waterTank/geometry/setYLength      50 cm
> /gate/waterTank/geometry/setZLength      50 cm
> /gate/waterTank/setMaterial              Water
>
> The above commands place the water phantom centered at (0,0,25). I
> then put a source at (0,0,-100cm), i.e. a source coming from the
> negative z-axis towards the positive z-axis:
>
> /gate/source/addSource mybeam gps
> /gate/source/mybeam/gps/ene 20 MeV
> /gate/source/mybeam/gps/particle gamma
> /gate/source/mybeam/gps/ene/type   mono
>
> /gate/source/mybeam/gps/position       0 0 -100 cm
> /gate/source/mybeam/gps/pos/type       Point
> /gate/source/mybeam/gps/ang/type       iso
>
> /gate/source/mybeam/gps/ang/mintheta   177.1376 deg
> /gate/source/mybeam/gps/ang/maxtheta   180 deg
> /gate/source/mybeam/gps/ang/minphi     0 deg
> /gate/source/mybeam/gps/ang/maxphi     180 deg
>
> According to the gps documentation theta = 180 deg corresponds to a
> beam travelling towards positive z-axis. Therefore, in order to form a
> divergent beam that enters the water phantom as a 10x10cm field, I
> need to restrict theta precisly atan(5/100) * 180 / pi = 2.8624 deg.
> That is where 177.1376 comes from.
>
> I've looked at my geometry (X-Y, X-Z, Y-Z planes) and it looks ok.
>
> Finally, I put my dose actor at (0,0,0) which is along the central
> axis of the phantom since z=0 is the entry point of my beam and the
> whole phantom is centered at (0,0,25).
>
> /gate/actor/addActor                               DoseActor
>  doseDistribution
> /gate/actor/doseDistribution/attachTo              waterTank
> /gate/actor/doseDistribution/stepHitType           random
> /gate/actor/doseDistribution/setPosition           0 0 0 cm
> /gate/actor/doseDistribution/setSize               1 1 20 cm
> /gate/actor/doseDistribution/setResolution         1 1 2000
>
> (for brevity I skip the commands that are not related to geometry)
>
> The dose actor will output 2000 values that correspond to the 20cm
> worth of water phantom (I suppose). The first value corresponding to
> depth z=0, the 2nd value for z=0.01cm and so on. Is that correct or do
> I have to specify some sort of direction to the actor ?
>
> Also the one million dollar question. The dose profile that I get does
> not make sense to me at all. I'm not seeing any build-up region. Here
> is the plot:
>
> http://dl.dropbox.com/u/25202446/opengate2.png
>
> Does that make sense to you ?
>
> Thank you for your patience and I promise I'll write up a nice
> tutorial for novices like me when I master Gate!
>
> Best regards,
> Stathis
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>
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