<div dir="ltr"><div dir="ltr">Dear Sara<div><br></div><div>It looks like the cross section for air in Geant4 and EGSnrc are diferent. EGSnrc has a good perform at low energy, then it is normal that for high energy both software match. </div><div><br></div><div>Another different in the stepfuntion, I mean the algorithm is different. Now as Tony said, you can change the step size, it is a tricky way to chance the cross section for an specific material.</div><div><br></div><div>If you want to go in deep, can be good check the physic manual of each code and ask directly in the Geant4 and EGSnrc blogs.</div><div><br></div><div>Best regards</div><div>Maikol</div><div><br></div></div></div><br><div class="gmail_quote"><div dir="ltr">El mié., 31 oct. 2018 a las 11:19, Sara Neira Castro (<<a href="mailto:sara.neira.castro@hotmail.com">sara.neira.castro@hotmail.com</a>>) escribió:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<div>Dear Gate users,<br>
<br>
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).
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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:
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<div># GEOMETRY<br>
/gate/world/daughters/name cajaAgua<br>
/gate/world/daughters/insert box <br>
/gate/cajaAgua/geometry/setXLength 29 cm <br>
/gate/cajaAgua/geometry/setYLength 29 cm <br>
/gate/cajaAgua/geometry/setZLength 30. cm <br>
/gate/cajaAgua/setMaterial Water<br>
<br>
/gate/cajaAgua/daughters/name aire1<br>
/gate/cajaAgua/daughters/insert box <br>
/gate/aire1/geometry/setXLength 29 cm <br>
/gate/aire1/geometry/setYLength 29 cm <br>
/gate/aire1/geometry/setZLength 3. cm <br>
/gate/aire1/setMaterial Air<br>
/gate/aire1/placement/setTranslation 0. 0. -10.5 cm <br>
<br>
/gate/cajaAgua/daughters/name aire2<br>
/gate/cajaAgua/daughters/insert box <br>
/gate/aire2/geometry/setXLength 29 cm <br>
/gate/aire2/geometry/setYLength 29 cm <br>
/gate/aire2/geometry/setZLength 3. cm <br>
/gate/aire2/setMaterial Lung<br>
/gate/aire2/placement/setTranslation 0 0. -4.5 cm <br>
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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).
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<br>
# SOURCE<br>
/gate/source/addSource fuente gps <br>
/gate/source/fuente/gps/particle gamma<br>
/gate/source/fuente/gps/energytype Mono<br>
/gate/source/fuente/gps/monoenergy 200 keV<br>
<br>
/gate/source/fuente/gps/type Plane<br>
/gate/source/fuente/gps/shape Rectangle<br>
/gate/source/fuente/gps/halfx 2.5 cm<br>
/gate/source/fuente/gps/halfy 2.5 cm<br>
/gate/source/fuente/gps/centre 0. 0. -15 cm<br>
<br>
/gate/source/fuente/gps/angtype iso<br>
/gate/source/fuente/gps/mintheta 0. deg<br>
/gate/source/fuente/gps/maxtheta 0. deg<br>
/gate/source/fuente/gps/minphi 0. deg<br>
/gate/source/fuente/gps/maxphi 360. deg<br>
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Thank you very much in advance, <br>
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<div>Sara </div>
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