[Gate-users] Dose in Air

[Sara Neira Castro]

Dear Gate users,

We have recently started to use Gate to simulate dose deposition in phantoms/CTs, and we have found some inconsistencies in doses deposited in low  density regions (mainly air) when crosschecking with EGSnrc: doses calculated with Gate are much lower than doses calculated with EGSnrc. In the  figure we report PDDs for different energies (photons) in a phantom formed by several layers of material (Water-Air-Water-Lung-Water). Material definitions are identical for Gate and EGS. Doses in water are identical, and doses in lung are very similar, but doses in air differ for low energies.  Interestingly, this effect seems to vanish with increasing energy (>1 MeV).

We have checked the input thousands of times, and yet we cannot find any explanation. We have tried different Gate physics packages (emstandard_opt3,  emstandard_opt4,penelope ...) and cutoffs (typically 10 keV or below), but the systematic difference is still there.  Is there any known limitation of Gate for simulating dose deposition of low energy photons in low density media?, or is it some kind of error in our simulation?. We would be grateful for any suggestion. Below are the main features of the Gate code:

# GEOMETRY
/gate/world/daughters/name     cajaAgua
/gate/world/daughters/insert   box
/gate/cajaAgua/geometry/setXLength 29 cm
/gate/cajaAgua/geometry/setYLength 29 cm
/gate/cajaAgua/geometry/setZLength 30. cm
/gate/cajaAgua/setMaterial Water

/gate/cajaAgua/daughters/name     aire1
/gate/cajaAgua/daughters/insert   box
/gate/aire1/geometry/setXLength 29 cm
/gate/aire1/geometry/setYLength 29 cm
/gate/aire1/geometry/setZLength 3. cm
/gate/aire1/setMaterial Air
/gate/aire1/placement/setTranslation 0. 0. -10.5 cm

/gate/cajaAgua/daughters/name     aire2
/gate/cajaAgua/daughters/insert   box
/gate/aire2/geometry/setXLength 29 cm
/gate/aire2/geometry/setYLength 29 cm
/gate/aire2/geometry/setZLength 3. cm
/gate/aire2/setMaterial Lung
/gate/aire2/placement/setTranslation 0 0. -4.5 cm


The source consists in a plane of monoenergetic photons which  is located on the front face of the geometry.  The code was run multiple times for several energies (200, 511, 1000 and 3000 keV).

# SOURCE
/gate/source/addSource fuente gps
/gate/source/fuente/gps/particle gamma
/gate/source/fuente/gps/energytype Mono
/gate/source/fuente/gps/monoenergy 200 keV

/gate/source/fuente/gps/type Plane
/gate/source/fuente/gps/shape Rectangle
/gate/source/fuente/gps/halfx 2.5 cm
/gate/source/fuente/gps/halfy 2.5 cm
/gate/source/fuente/gps/centre 0. 0. -15 cm

/gate/source/fuente/gps/angtype iso
/gate/source/fuente/gps/mintheta 0. deg
/gate/source/fuente/gps/maxtheta 0. deg
/gate/source/fuente/gps/minphi 0. deg
/gate/source/fuente/gps/maxphi 360. deg


Thank you very much in advance,
Sara

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