PPD issue in water phantom simulation (photon beam)

Dear All relying on medical_linac example I’ve developed very simple example with cubic water phantom and primary particles (photon beam) from iaea phase space file.
This is a minimal version of more advanced project that I would like to continue…
However, I found an issue with physics characteristic - dose profile, PDD, comparing with the measurement and other simulation s/w:

I’ve detailed description and full code in the repository:

Can anybody point me, what can be wrong in this implementation?

Dear all,
to better describe this issue I’ve prepared few slides: SIM-PRIMO-IAEA-G4-Exercise.
I would be very grateful if experts would take a look…


I could not see all the curves indicated in the legend in the plot. So what you are saying is that Geant4 produces a PDD which is very different from what expected. I find this result very strange as usually Geant4 re-produces fairly well PDDs for clinical X-ray beams.
Do you adopt the same voxel size in all the calculations that you are comparing?
For testing, could you use as input the IAEA files in the Gamma Therapy extended example and calculate the dose in a water phantom? Then I suggest to use G4EmStandard_opt3 or 4.

Dear Guatelli,
Thank you for your hints! You inspired me to dig into the measurement procedure - how the profiles are being measured. My problem was related to this misunderstanding - the measure volume in the water phantom is tiny comparing to the size of the radiation field or phantom size (probe of few millimeters).
I’ve been scoring the dose (e.g. the PDD) from the whole phantom volume:

/score/mesh/boxSize 24. 24. 17.5 cm

instead I should use the size smaller than the actual field size:

/score/mesh/boxSize 1. 1. 17.5 cm

After this change I’m getting reasonable results…
Thank you!


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excellent! could you please mark as “resolved”?