Date: Mon, 10 Apr 2017 05:20:33 +0000
Dear Stefan,
Thank you for your attention. I believe my question was more related not to Fluka but the idea of dose rate constant - the one used in brachytherapy protocols such as TG-43. However, If I understand correctly the dose rate constant is the ratio which can be used to check the accuracy of the simulation itself, so it would be nice to use Fluka at least for the comparison with MCNP which is more widely used in the medical physics. The funny thing is that suppose to be smth very simple but several professionals including the ones working in the field could not explain this to me - but maybe this is my problem, don't know.
The dose rate constant Λ (cGy/U-hr) is defined as the dose rate to water at a distance of 1 cm on the transverse axis of a unit air kerma strength source in a water phantom.. So:
I scored the dose from the seed in the sphere with water medium and air medium, see attached.
1) the Ir192 seed's dose rate constant is confirmed as ~ 1.1 cGyh-1 U-1 (it's activity is around 400 GBq) as reference.
2) if I normalize the Gy per Bq value to 4e11*3600 I get the air unit dose at 1m around 0.036 Gy/h (pretty close to the expected)
3) the dose from the seed in the water at 1 cm is, however, ~160 Gy/h
So how to relate these quantities to get the reference dose rate constant - 1.1 cGyh-1 U-1 ?
I understand that there is smth I don't see and maybe the whole approach is wrong yet learning curve is always steep without any good examples or expert's guidance.
________________________________
From: Stefan Roesler <sroesler_at_mail.cern.ch>
Sent: Sunday, April 9, 2017 11:27:49 PM
To: Ševčik Aleksandras
Cc: Alfredo Ferrari
Subject: Re: [fluka-discuss]: 192Ir isotope dose rate constant
Dear Alex
Maybe tell exactly which value from which scoring card you used for the
100cm and 1cm doses.
For example, I see a scoring by region. Note that in this case FLUKA does
not divide by the volume of the scoring region, which you'll have to do
yourself offline.
Cheers
Stefan
On Tue, 28 Mar 2017, ?ev?ik Aleksandras wrote:
>
> Dear experts,
>
>
> In my pursuit to calculate the 192Ir dose distribution in the Zubal head
> phantom using Fluka code I got stuck again - this is time in calculating the
> dose rate constant. As no external help available for me locally, I would
> appreciate any insights or comment from you.
>
>
> As it is stated, the dose rate constant ? (cGy/U-hr) is defined as the dose
> rate to water at a distance of 1 cm on the transverse axis of a unit air
> kerma strength source in a water phantom.
>
> So it is necessary to measure both the reference air kerma rate at 100 cm
> and the absorbed dose rate to water at 1 cm distance from the source
> perpendicular to the source axis
>
> I modelled the seed geometry and usrbin-ed the dose values in 150 cm radius
> sphere, filled with air and later- filled with water. In the air sphere the
> dose rate at 100 cm from the seed approx. 2.7E-17 Gy per decay that more
> less is what can be expected (400GBq would give ~39 mGy/hr in comparison
> with expected ~45 mGy/hr)
>
> The dose rate in the water sphere at distance 1 cm from the seed surface is
> around 1.2E-13 Gy per decay. So here I stuck - ther ref value for dose rate
> constant for 192Ir seed should be approx. 1.1 cGy/(U-hr). How to combine all
> this data to something like that?
>
> I assume there is a certain "obvious" thing I am missing, due to lack of
> experience.
>
> Input files are attached in case it is interesting. Dose rate values are
> scored in usrbin R-phi-z mesh then plotted as type 1D Max in Flair on R
> axis. However I tried many other options including setting detectors
> regions, etc. I also noticed that energy spectra is missing ~1.4 MeV spike
> in comparing with spectra from MCNP but maybe this is not related or there
> is other explanation for this.
>
> Regards
> Alex
>
>
>
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(image/jpeg attachment: 192Irairwater.jpg)