[Gate-users] Mean energy vs. Radial distance in Elekta Precise 6MV phase-space data

[Stathis Kamperis]

On Mon, May 12, 2014 at 11:40 AM, Grevillot Loic
<loic.grevillot at medaustron.at> wrote:
> Hi Stathis,
>
Hi Loïc and thank you for being so kind as to answer my questions!

>
>
> What you should observed is the following:
>
> -          At the center: a higher energy and a lower fluence due to the
> filter thickness
>
> -          With increased radial distance: a reduced mean energy and
> increased fluence
>
> -          At the filter border: you suddenly have no filter material, so
> the fluence increases and the mean energy decrease.
>

Indeed, all these make sense. As thickness increases, fluence
decreases due to attenuation and mean energy increases due to beam
hardening.

What puzzled me the most though, was the sudden peak of mean energy
near the flattening's filter edge. I was expecting the curve to
decrease monotonically.

I double checked my graph (with R and Mathematica) by looking at ~1.5
x 10^6 photons. The peak is always there.

I have a theory, but this is just a theory :)

http://dl.dropboxusercontent.com/u/25202446/peak-theory.png

> Actually, what happens at the border of the filter is not important, in the
> sense that this part of the beam is not use clinically.
>

Indeed. I was just curious from a purely scientific point of view :)

Thank you again!

Best regards,
Stathis

>
>
> Sincerely,
>
>
>
> Loïc
>
>
>
>
>
>
>
> From: gate-users-bounces at lists.opengatecollaboration.org
> [mailto:gate-users-bounces at lists.opengatecollaboration.org] On Behalf Of
> Stathis Kamperis
> Sent: Freitag, 9. Mai 2014 19:15
> To: gate-users
> Subject: Re: [Gate-users] Mean energy vs. Radial distance in Elekta Precise
> 6MV phase-space data
>
>
>
>
> Στις 9 Μαϊ 2014 10:28 π.μ., ο χρήστης "Stathis Kamperis"
> <ekamperi at gmail.com> έγραψε:
>>
>> Hello everyone,
>>
>> I'm using the Elekta Precise 6MV phase space data from the site of
>> IAEA. I was plotting the mean photons' energy vs. radial distance and
>> there's a peak around ~6-7.5cm that I cannot explain:
>>
>> http://dl.dropboxusercontent.com/u/25202446/peak.png
>>
>> Also, the graph from here
>> http://www.creatis.insa-lyon.fr/~dsarrut/articles/Grevillot2011.pdf
>> lacks such a peak (not sure if it is the same exact linac model).
>>
>> The closest clue I could get is from Grevillot et al:
>>
>> "... step in the particle radial fluence is observed between 65 and 70
>> mm in (b), corresponding to interactions of the photons with the flat
>> bottom edge of the flattening filter."
>>
>> Do you have any suggestion on how to interpret my results ? Perhaps
>> the simulation of the IAEA phase-space data used a smaller flattening
>> filter ? And too many photons pass through its edge ?
>
> Actually that doesnt make sense. We are plotting mean energy, not fluence.
>
> (I think that
>> the plot code is correct, since it uses the fields package of R.)
>>
>> Thank you for your consideration,
>>
>> Best regards,
>> Stathis
>
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