Help with F4 Fm4 dose calculation in MCNP simulation

[alinegranja]

TL;DR Summary

Struggling with dose calculation in MCNP for brachytherapy using Ho-166. Using Fm -1 0 -5 -6 for photons results in zero, unlike F6/F8. Need help configuring F4:e,p with Fm4 to accurately calculate deposited dose for both photons and electrons.

Hello everyone,

I am facing difficulties while trying to calculate the dose in the tibia due to brachytherapy in an MCNP simulation. We are working with the radionuclide Ho-166, and therefore, we need to account for both photon and electron contributions to the deposited dose.

Initially, I used the card Fm -1 0 -5 -6 for photons, but the result was zero, compared to the results obtained with the F6 and F8 cards. I would like to better understand the correct way to configure the dose calculation in the region of interest (tibia). Additionally, I have not found a clear reference for how to use the F4(FMESH4) card with the :e,p specification to properly calculate the dose with the Fm4 card.

Could someone help me understand the correct configuration to obtain the expected results for deposited dose in MCNP, considering both photon and electron contributions?

Thank you in advance for your help!

 

[Alex A]

Welcome to Physics Forums @alinegranja,

It is my current understanding that...

If the target is not modeled in the input file, DE/DF cards can be used to get the expected dose from the flux. Prior to 6.3 there are even built in functions. This would be expected full body dose and not what you want I think, but it might be worth double checking the answers with these numbers. They should be close.

If the target is present and accurately modeled (the shape need not be exact), the density is right, the total masses are right then an F6 tally would report how much energy is deposited in the cell. Using *F6 would give the answer in jerks per gram (per source particle), multiplying that by 10^12 should then give you Joules per kg (per source particle). Taking that value and multiplying by the activity and the exposure length should then give a dose in Grays.

The FM card is usually used to multiply the flux in a tally by a cross section and integrate, for example to find the rate a reaction happens. I don't understand why it would be used here.

I hope this helps, and best of luck working it all out!

 

[alinegranja]

Hi Alex...

The issue is that I actually need to create the dose map for the tibia, which passes through the defect (tumor), the trabecular bone, and finally the cortical bone. For this, I am using the Fmesh4 tally, as I need to calculate the dose at various points along the tibia's dimension. However, when applying the DE/DF card, I encounter an issue due to the overlap of different materials. In summary, I need to use Fmesh4 with the FM applied to different materials. Below is an example of the geometry input:

c BLOCO 1----------------------------------------------------------------------
c Tibia -----------------------------------------------------------------------
c Osso esquelético
10 3 -1.85 -100 300 imp:p=1 imp:e=1
c Osso cortical
20 2 -1.85 100 300 -200 imp:p=1 imp:e=1
c Defeito
30 1 -0.998 -300 imp:p=1 imp:e=1
c
c Universo vazio
999 0 200 imp:p=0 imp:e=0
c -----------------------------------------------------------------------------

c BLOCO 2 ---------------------------------------------------------------------
c Superfícies
c Osso esquelético
100 RCC 0 0 0 40 0 0 3.2 $ c=(0,0,0) h=40 cm //x r=3.2 cm
c Osso cortical
200 RCC 0 0 0 40 0 0 3.5 $ c=(0,0,0) h=40 cm //x r=3.5 cm
c Defeito quadrado 1 cm³
300 RPP 19.5 20.5 2.4 3.4 -0.5 0.5
c -----------------------------------------------------------------------------

...

I want to calculate the dose in a mesh with the following specifications:

x: 0 to 40 with 800 divisions
y: -3.5 to 3.5 with 140 divisions
z: -0.05 to 0.05 with 1 division
So, the resulting voxels would be 0.5 x 0.5 x 1.0 mm.

I was hoping to associate the Fmesh4 with the FM card in order to obtain the dose. However, I have not been able to use DE/DF because the mesh passes through different tissues.

 

[Alex A]

What version of MCNP are you using?

 

[alinegranja]

What version of MCNP are you using?

6.2

 

[Alex A]

I see what is happening with the FM card, it's a multiplication by the cross section to get the collision rate and by the average energy per collision to get the dose. I don't think it can automatically do multiple particles because the special reaction numbers are all different. I wonder if the method for electrons will even work at all. I do not know why it is failing for photons.

With MCNP6 you can use a type 3 TMESH. This should tally all deposited energy and you should not need to specify which particles.

I assume something like,

tmesh
rmesh13
cora13 0. 799i 40.
corb13 -3.5 139i 3.5
corc13 -0.05 0.05
endmd

will work. This is in MeV/cc.sourceparticle. Note - per cubic centimeter, not per gram. I read that an FM card (which you might want to convert to rem or grays) is not allowed with the type 3 tmesh so conversion may need to be done another way.

It may be possible to use a type 1 tmesh with the PEDEP keyword AND an FM card. If PEDEP permits multiple particles.

This was more complicated than I expected. Hopefully if I've missed something or made a mistake someone will correct me.

Good luck!

 

[alinegranja]

Thank you, Alex!

I was able to obtain the dose using TMESH, but I am still facing the issue with considering the different materials. Since TMESH accounts for the dose in MeV/cm³, it does not take into account the influence of the different tissue densities, which is a problem in my calculation, as density directly affects the dose response (comparing the results +F6, *F8, and TMESH, the TMESH results are not compatible with any of them).

I will continue searching for a solution to integrate the material density into the simulation, in order to obtain a more accurate dose.

Thank you again for your suggestions, and any further help will be greatly appreciated!

 

[Alex A]

I am out of my depth, especially on a medical physics question. Is it important that the measured dose is based on the energy deposited in the simulation, or would a dose response curve do?

If you need it done based on deposited energy, most voxels in the mesh will only contain one material. These can be converted because the density is known. I don't know an easy way of which material is in a voxel and doing the conversion.