@bellasdad
I was also curious about the same questions. I was treated on the MRIdian platform and at the end of my final treatment I asked the nuclear physicist that was there for each of my five treatments if she could tell me the answers to some of the items you questioned. So she was very interested in telling me all about it, I think she was happy that a patient was interested. So, yes in my case they treat the whole prostate each session. I had five and in each 5 they used 17 different scan angles to achieve their goal. When I did mine I asked from the very beginning if I could watch the imaging live on the TV monitor in the scan room instead of the nice soothing nature images displayed to relax you during the treatments and they said sure no problem, so what they were seeing I got to see each time. What it showed me was my prostate with the 2mm border live and I could witness my prostate move at times, but the movement never went outside of the border. The physicist then showed me on her monitor each of the 17 beams were rectangular and the program broke down the coverage area. She explained how that beams were also adjusted in intensity to be of higher concentration over lesions and lesser of my urethra and other non cancerous areas.. etc. But she made the point to say that all of the prostate is treated but in a very specific and controlled way. I wish I had more to spend with her, it was very interesting and I could have learned more. My total at the end was 42.5 gy. Hope that helps, but were all different.