[Gate-users] Validation with TLDs

[BAKER, Mark (THE CLATTERBRIDGE CANCER CENTRE NHS FOUNDATION TRUST)]

Hi all

This question isn't specifically about GATE, but it is a task I need to do in order to continue using my GATE macros - the dreaded validation. I'm sure other people might have ran into the same issues at some point, or might have a better solution.  My code is intending to work out dosimetry in a voxelised PET-CT scan, and I was originally going to validate my code by sticking some LiF TLDs on the outside of a phantom containing F18. The phantom would then be PET-CT scanned to generate the CT and PET data for the simulation, then left for a period of time to allow the TLDs to accumulate dose. I'm aware that there is then a component of CT dose to the TLDs, but with a low enough CT exposure and large enough activity, I am hoping that this dose just washes out in the overall errors.

I don't know the calibration of the TLDs (they are RT TLDs so are used more for MV than PET) so I put a bunch of them at various distances from a known activity of F18 in a vial for a known time period, and measured the dose rate and accumulated the total dose. By plotting the light output against the known dose, I could then measure the light output of my validation TLDs to determine their dose and compare it to the dose modelled by GATE.

The problem I am having is that obviously the TLDs have a component of positron dose alongside the 511 keV annihilation photons. My dose rate was calculated using a Fluke Raysafe 452 which has a filter to convert to a dose rate, which I suspect blocks the majority of the positrons, underestimating the dose rate.

Has anyone else validated their positron/photon dose combined or have any suggests on ways I could make this better?

Thank you

Mark Baker
Principal Clinical Scientist
Imaging Physics (Ionising)
The Clatterbridge Cancer Centre NHS Foundation Trust
(he, him)

CCCW Tel:  0151 556 5030
CCCL Tel: 0151 318 8438
Email: mark.baker23 at nhs.net<mailto:mark.baker23 at nhs.net>
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Honorary Lecturer, Dept. of Physics
University of Liverpool

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