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

[Stathis Kamperis]

Thanks Loïc! I forgot the perhaps most fundamental rule that
everything is positioned at the center of mother volume! It works fine
now! (fine as in I see the build up region; my Dmax'es are still off
and under investigation).

Best regards,
Stathis

On Mon, Feb 3, 2014 at 7:19 PM, Loic Grevillot <loic.grevillot at gmail.com> wrote:
> 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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>
>