@johnoclark
I have provided a few links that may be helpful. Ask your radiation oncologists about the margins the machines use as they do impact health tissue. I had 5 hypo fractional treatments on the MRIdian machine but as you will read, studies are showing that 2 hypo fractional treatments may do the job. Where I was treated, they had both the Proton radiation and the Mridian machine available locally for me.

The link below is to Elekta's MRI guided MRlinac machine. You will also see information on four MRI guided machine studies: the Mirage, Momentum Hermes and Erect. These should give you a good feel for the advantages of an MRI guided radiation machine that is real time vs fused images.
https://www.elekta.com/products/radiation-therapy/unity/prostate/
The link below is to information regarding the Mridian machine as explained by UCLA Health regarding the MRI guided MRlinac machine.
https://www.uclahealth.org/cancer/cancer-services/radonc/cutting-edge-devices/viewray
The link below is to Proton therapy radiation as explained by MD Andersen
https://www.mdanderson.org/patients-family/diagnosis-treatment/care-centers-clinics/proton-therapy-center/what-is-proton-therapy.html
Once I understood the issue of margins and real time MRI guidance, the other radiation machines that did not have real time imaging fell off my list of possibilities. Proton stayed on as one of two final choices for me because of its dosimetry calculation of the Proton energy stopping at a specific level though the energy in other ways was still moving and the margins that I discussed at the time were 3-5 mm vs 2mm with real time imaging.