# Microdosimetric spectrum (Y - YDY)

Dear colleagues.

There is a problem of determining the microdosimetric spectrum of the distribution of linear energy (Y) in water. (Y - YDY).
Y - linear energy transfer (keV/um)
YDY - is the dose probability density (DY) multiplied by linear energy transfer(Y).
I am attaching an example chart!

I do not understand how I should build the geometry of the phantom and sensitive detectors, so that such a graph. Who did this task? tell me a few steps pls?

Dear Anatoly,

it is possible to calculate microdosimetric spectra with Geant4. For example look at this paper:
D. Bolst et al, Validation of Geant4 for silicon microdosimetry in heavy ion therapy, Physics in Medicine and Biology], Volume 65, Issue 4, 13 February 2020, Article number 045014.

The advanced example radioprotection calculates the energy deposition spectrum. Then you can divide the edep the by the chord length and find the spectrum of y. Finally in post-processing you can calculate the microdosimetric spectra (please, remember to use the log scale on the x axis).

I hope this helps.

Cheers
Susanna

Dear Susanna,
Thanks for this information!
So, in the water phantom, I built 20 sensitive detectors (diameter 1 μm).
This way I get edep for each microdetector!

But I don’t understand how to calculate yd(y)!!!

1111|483x428

beat regards
Anatoly

Dear Anatoly,

I will write here some notes on how to perform experimental microdosimetry calculations via Geant4.

• The energy deposition is calculated for a track incident on the SV (either primary or secondary track). If the track generates secondaries in the SV (e.g. delta electrons), their contribution should be added to the traversing track.
• I do understand that you integrate the energy deposition spectra over the SVs.
• Then you should divide the energy deposition counts by the chord length. Now depending on your radiation field, you may want to consider the average chord length as provided by Cauchy’s formula (case of an isotropic field) or calculate it yourself. This step is necessary if the radiation field is not isotropic.
• For the microdosimetric spectra, plot y*d(y) against the log of y. I suggest to read the section on standard plotting method in Peter Bradley’s thesis, available at https://ro.uow.edu.au/theses/1367/
• Remember then to normalise the area of the microdosimetric spectra to 1.

Please note that to calculate microdosimetric spectra from energy deposition spectra in the SVs is independent from the Geant4 simulation as it is part of post-processing analysis. We will provide the methodology to calculate experimental microdosimetric spectra in next future developments of the radioprotection advanced example.

I also suggest to you to look into the microdosimetry example.

Cheers
Susanna

Dear Anatoly,
there are several publications on calculating microdosimetry spectra / variables with a Geant4-based application, please follow the links for details:

https://doi.org/10.1088/0031-9155/60/8/3313

Sure, you have to implement your own microdosimetry detector (Rossi-type or wall-less etc.) A good guiding manual is ICRU report on microdosimetry.