Dear @Jungle,
I see in the input that you have ignored most of the advice given in our previous discussion.
Let me then be more explicit:
The image above results from a simulation that took 76 CPU minutes in my laptop, as opposed to the 1360000 CPU minutes that would take to obtain the results you have reported above using the same number of primaries you have used, also on my laptop. Note the factor ~18000 between both. Those results are obtained by implementing the three points of my initial reply and reducing the threshold, as proposed in my second reply.
So, let me stress once again that the use of importance biasing here is pointless and detrimental. You are splitting photons as they cross regions, effectively simulating more photons, wasting CPU time that would be better employed simply simulating more primary photons, which have completely uncorrelated histories, as opposed to what happens with the ones generated via splitting.
On a secondary note and for completeness (hopefully not diverting your attention from the main point above): the patterns are a consequence of the transport cuts, in combination with your USRBIN and geometry. I ignore the technical details giving rise to them, but I suspect they are related to the influence of boundaries on the transport of electrons approaching them. The consequences of this are very much reduced by lowering the energy cuts, as you have observed. Once this is done, my advice would be to remove the futile region subdivision to avoid any potential impact of such boundaries, which is anyhow my initial advice.
I hope this helps,
Francisco
PS1: I see the dose map in your plots is elliptical, which I believe would be the case if no Co is filling your source region. I assume you have run those simulations without it.
PS2: attention to the FUDGEM parameter when defining the production thresholds!