Because cylindrical geometry is involved, using a cylindrical FMESH tally would be better. Adjust those boundaries such that points in mesh tallies lie on the materials' interface. Use different mesh tally for each region and particle type

Hi Aline,

First of all be careful when using DE/DFs for calculating dose. Those conversion factors are for calculating effective dose and therefore are not applicable to medical dosimetry where the interest is in dose (energy deposited per mass).

I am assuming that the brachytherapy source you are simulating emits both photons and electrons. Why don't you run an MCNP case with just photons and one with just electrons? You can then sum the results to get a dose estimate. With that being said a +F6 can be more appropriate because depending on the geometry and the material an F6:p, e may double count the electrons coming from secondary reactions.

My suggestion would be to calculate the dose for cells (rather than splitting the geometry in thinner pieces) using the different tallies compare the results and if they make sense continue with the splitting. Tally segmentation can also assist although it is kind of tricky to make sense of how to use them in 2-D slices. They worth a try.

I hope I helped with some inspiration.

Good luck,

dimitris

Hi Aline,

You can read the first pages of MCNP 6 Advanced Tallies Tutorial (2014). As Dimitri said the +F6 seems the best

Hi Dimitrios Kontogeorgakos ,

Thank you very much for your response and suggestions. When you mention calculating the dose for cells before segmenting the mesh, are you suggesting dividing the geometry into multiple voxels and assigning a specific tally to each one instead of using a MESH?

From my studies, I saw that TMESH is similar to +F6, so I considered using it. However, my biggest doubt is whether +F6 or TMESH already take into account the material (density) where the dose is being calculated, or if I need to perform this conversion myself (and how to do it). I couldn’t find a clear explanation in the manual… Do you have any insights on this?

Thanks again for your help!

Hi Aline Granja,

The best approach depends on the geometry and the locations where you need to calculate the dose. Since this is related to brachytherapy, selecting the appropriate cells for dose calculations should be straightforward. A logical starting point is to define cells near the source. If these cells are large relative to the particles’ energy range, consider refining the volumes to better capture energy deposition at different depths.

While voxel-based geometry is indeed a strong choice, beginning with a simpler approach can provide valuable insight into how energy deposition depends on cell size. This method allows you to refine your model iteratively before committing to a more complex voxelized representation. However, this is how I would approach the problem—the final decision is yours.

Regarding TMESH, it is superimposed over the problem geometry. If you are using PEDEP to obtain energy deposition values, keep in mind that the output is given per cm³ and is independent of material density. To obtain energy deposition per gram, you must divide the results by the material density. Additionally, be mindful of where the mesh is superimposed—if it spans multiple materials, you may need to exclude or adjust results accordingly.

Regards,

dimitris

Hey everyone,

Thank you all for your contributions to this discussion! Your input has been incredibly valuable.

I’m leaving a summary of our findings here for anyone who might visit this discussion in the future:

In medical dosimetry simulations, it is essential to verify whether the material is properly accounted for in the dose calculation performed by the code. Through a series of tests, we confirmed that TMESH can be used to map dose distribution in a complex geometry with multiple materials.

However, during our analysis, we found that the code does not have a direct multiplier to convert dose from MeV/cm³ to MeV/g when different materials are involved. This conversion would require dividing the original TMESH output by the density of each material. If such a conversion is needed for a simple and well-defined geometry, we can develop a script to handle it.

Key points:

• TMESH type 3 returns the dose deposited by all particles (equivalent to +F6) in MeV/cm³.
• To check whether the material density is being considered in the calculation—i.e., to obtain MeV/g from TMESH—one can compare it to the +F6 tally result.
• Important note: When performing the calculation, you must consider the volume of the MESH where the dose is being computed, not the volume of the cell.
• To confirm this, set up a simulation using +F6 and create a TMESH with the same dimensions as the cell where +F6 is being calculated. Then, convert and compare the results.

Our next challenge is to determine whether it is possible to parallelize processes in a simulation using TMESH. Here’s the link to that discussion:

https://www.researchgate.net/post/Can_TMESH_be_used_with_tasks_in_MCNP6

Regards,

Aline