# Simulate voxel S Value on PET-CT

Dear experts:
I already have DICOM data of the patient’s F-18 and have imported it into FLUKA to obtain a VOXEL file. In order to calculate the dose of F-18 in the patient’s body, referring to MIRD’s method for dose calculation, I first need to obtain the voxel S values of different organs. How can I simulate to obtain the energy spectrum, absorption fraction SAFs, and voxel S value of F18?

Dear @dengtiantian22

I am not familiar with the S-value, however from the K. Khamwan, A. Krisanachinda, P. Pasawang paper, if my understanding is correct, the S-value is used to convert the Activity (Bq) to Dose (Gy), such as D = A \times S
using an approximate formula to calculate the S from the decays, and the fraction that is absorbed per organ (Eq, 1.,2,3).

If so, FLUKA can simulate for you the decays, their absorption, so you don’t need any approximation.

You could do a simulation:

• using as geometry the CT scan converted to voxel (as you already did)
• convert the F-18 PET activity as a USRBIN. In flair select the CT scan and click on the Export dropdown menu and select USRBIN
• Add a SPECSOUR card with Type: BIN-SOUR and select the PET USRBIN you created before (Using also an OPEN card to load the file).
• In the BEAM card select ISOTOPE and with HI-PROPE card select the decaying F-18 isotope
• WIth a USRBIN matching the VOXEL (From flair DICOM, select the CT and add USRBIN card) score DOSE

If you divide the simulated DOSE with the USRBIN activity of F-18 you should get the S-value per voxel. You could even sum it up to get it per organ, maybe using the RTSTRUCT information.

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Hello @vasilis :
Thank you for the above detailed steps, following the steps you gave, I got an error after running it.
and I still have some questions as follows:

1. Do I need to set the energy of the BEAM when I calculate the Dose?
2. RTSTRUCT includes a lot of brain ROIs, if I want to calculate the absorbed dose of the brain, do I need to use BEAMPOS to set the location of the source? In the picture, you can see that my BEAM didn‘t go through the brain tissue.

To describe my problem more clearly, I have attached my flair file.
F18.flair (3.4 KB)

Thanks and regards,
Dengtiantian.

Dear @dengtiantian22

1. Normally, you always need to set the maximum energy you will have in your simulation, in the BEAM to let FLUKA to initialize all internal tabulations. If you don’t it will take the default value of 200Gev/c
2. You don’t need any BEAMPOS with the BIN-SOUR option. However If you want to translate/rotate your USRBIN you need to use a roto-translation matrix in WHAT(3),(4) of SPECSOUR.

When using the BIN-SOUR flair is not able to display the BEAM position. If you want to visualize it add a USERDUMP card and dump all source particles.