I would say that my fatigue "wall" is pretty much a daily thing - if I want to be awake much after 8/9 pm I have to take a 30 min. or so nap mid-day.
If I don't it is just a point at which I'm out like a light. Either sit down or fall down, with little warning.
I am almost a year out from IGRT.
The RO told me that by two months out, I would start getting better. Said less than between the lines was that I would continue to get worse for those same two months. My GP thought that was optimistic. My GP was right.
Looked back at my notes on the immune system conversations. I mentioned that the RO dismissed it out of hand. My GP has a lot of various RT patients, and explained it, which I then reviewed with a friend who is a rad. tech. The summary of what they were both trying to tell me was this - photon RT hurts/kills everything in its path. They use the 7 positions so that those positions only get 1/7th of the kill every day, while to prostate area gets 7/7ths. Therefore the advantage to be offered by the rapid arc - more positions, less damage per line. Now, if/when large bone areas and blood vessels are in the path, they (the bones and the blood passing through at the time) get hurt too. So there is a certain kill-off of blood cells and bone marrow to be expected. In some people it is not so severe, in others it is more so.
In my case, blood test just a few weeks before RT started and those at the end showed a dramatic drop-off from my original "normal" counts in the numbers of both red and white cells. The red count came back up to almost minimums at about 4 months after RT ended, but for the white, it is just this last test (10 months out), and they are just close to, not at, minimums.
I understand the bummed out part. I am too. It has had a very significant impact on my job.