Dear @kwlibuaa,
Thank you again for your message.
The decreasing tail at the end of the LETt and LETd vs depth is the combination of the unresolved drop after the Bragg peak and the effect that, at the end of the curve where the fluence drops, numerical problems may emerge due 1/0 behaviour (also discussed in the post).
Related to the results from G4 you intend to reproduce, I understand they come from the following article: https://aapm.onlinelibrary.wiley.com/doi/epdf/10.1118/1.4932217.
In the figure 1 of that article, it is explained how fluence, LET and deposited energy are scored. Essentially, fluence and energy are scored in the thickness-dependant front layer of every voxel, while the deposited energy is calculated in the whole voxel. On the contrary, in your FLUKA input file, every variable is scored and averaged in the whole voxel. In particular, your simulation matches the case of 500um step thickness in G4 since the voxel size is 500um as well.
Related to your last comment:
In the section 3.B of the article, the author comments on the higher LETd values found for the case of 5um, which seem to appear due to small step sizes and disappear for larger steps. In FLUKA, the role of the step size is the same as for the large steps in the article. This is why all of your results are identical among them in FLUKA (step-size independent), and equivalent to those of 500 um in G4.
If you want to implement the alternative scoring methodology in FLUKA, you would need to have different USRBIN grids to score fluence, energy and deposited energy.
I hope this is helpful.
Best regards,
Mario Sacristan Barbero