How to use GPS in my project

Hello everyone,
I am currently using the human phantom (MIRD) example for my project which involves administering a dose of Iodine, making the center of the stomach as my source beam and getting the mean absorbed dose per organ at t=0 and t=30 min after the ingestion of the Iodine capsule. I was clueless to how I should go about simulating this but I have been told to define a GPS first, I found a file with some examples on GPS but I don’t seem to know which one is more suitable for my project, would you please take a look at the link below and see which one would be more suitable?

Thank you all for your help.
Best regards.


the advanced example Brachytherapy has macros that define a radioactive source in 3D using the General Particle Source. I think those macros would be a good starting point. The primary particle is the decaying parent radionuclide (iodine in your case, there is I-125 in he Brachytherapy example).

Thank you very much for your answer the macro for radionuclide decay is very helpful, I have also noticed the gps command in human phantom example can I use it instead while running the example?

Hi, yes, you can use it. It is simply that you can modify the UI commands to create your own primary radiation field of interest.

Thank you very much Susanna.
Let’s say that I have an initial number of particles or an initial dose (30 mCi) and that I want to run my event but said event has to stop in 15 minutes which means before all the matter has decayed, how should I do to get this result? In other words how can control time passage in the simulation?

Usually what we do in our group is to solve this problem analytically, because it is not possible to use the global time (it starts from zero with each event). You can use the radioactive decay equation and the activity to see how many radionuclides will decay in 15 min (N*).
You can do a simulation with a certain number of primary particles No. Then you divide the results by No and multiply by N*. That will be the dose given in 15 min. Here we do not take into account the biological half life.