Re: Medical linear accelerators

From: <Kristian.Ytre-Hauge_at_ift.uib.no>
Date: Wed, 06 Mar 2013 09:52:14 +0100

2013 09:52:20 +0100 (CET)
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Dear Vadim,

Thank you for you email.

I agree that there are many variable that may have an impact.

The SSD of 90 or 100 cm and changes in material density (especially
the flattening filter) seems to give changes up to about 10%. While
the differences in the literature are up to a factor 5.

The size of the electron beam is typically 1 mm FWHM or a bit bigger.
I have not investigated in detail the impact of this parameter with
respect to the number of primary electrons needed per Gy, but I
suspect that the impact of this parameter is relatively small compared
to the observed discrepancies?

Thanks again,
Kristian

Siterer

> Dear Kristian,
>
> Normalization factor depends on the size of an electron beam
> impinging on a target, a way how it was simulated (Gaussian or
> flat). The reference conditions may impact too, I.e. SSD of 90 or
> 100 cm. Perhaps yours was 90 cm as recommended, just to double check.
>
> Best Regards,
>
> Vadim
>
> Sent from my iPhone
>
> On Mar 5, 2013, at 8:55 AM, "Kristian Ytre-Hauge"
> <kristian.ytre-hauge_at_ift.uib.no<mailto:kristian.ytre-hauge_at_ift.uib.no>>
> wrote:
>
> Dear Fluka experts,
>
> I am simulating the neutron fluence and dose from a medical linear
> accelerator for radiation therapy.
> The principle of these accelerators are that an electron beam
> produces a wide photon field when hitting a target (typically
> Tungsten/copper target).
>
> In the first step of my simulation I am trying to establish the
> photon dose per primary electron under reference conditions,
> where reference condition means: dose in a small volume at 10 cm
> depth in water from a 10 x 10 cm2 photon field ++.
>
> When comparing my results to studies with other MC codes in the
> literature there are clearly large differences (for the same
> accelerator model and same settings):
>
> My work (FLUKA): 5.44 e14
> primary electrons per Gy [e-/Gy]
> Martinez-Ovalle et. al. (MCNPX, 2011): 2.72 e15 [e-/Gy]
> M.K. Saeed et. al. (Geant4, 2009): 1.25 e15 [e-/Gy]
>
> Because the neutron doses are reported per primary particle, the
> factors above are of great importance when we wish to report e.g.
> the neutron dose per treatment Gray.
>
> Does anyone have any suggestions on which parameters that may cause
> such large differences, or if differences of this magnitude is
> expected when using different Monte Carlo codes?
>
> My statistical error is 0.1% and no biasing has been used.
> I have used both the EM-CASCADE and PRECISION defaults (with
> similar results) and photonuclear reactions have not been activated
> when studying the photon dose.
>
> Due to confidentiality agreements with the vendor I am prevented to
> upload my Geometry.
>
> Any comments are welcome.
>
> Thanks,
> Kristian Ytre-Hauge
>
Received on Thu Mar 07 2013 - 17:58:35 CET