"Respected sources constantly recommend a plant-based diet"

They do, but they all recommend this for most anyone. It's like saying quit smoking, it's bad for you and quitting will imporve your overall health so you should do it to aid in improving your health as much as possible while going through this. I'm sure there are studies supporting it, but, more in improving general health. Does a plant based diet aid in direct treatment? .mmmm maybe. I'm pretty much on one as well, more because the healthier I am, the better to weather the treatment. It's the same reason you'll see a lot of folks, me included, advocating for a regular fitness schedule as well.

What the radiation oncologist will likely be more focused on is a diet that will help with your being ready for the treatment at the time of treatment, one that doesn't cause gas! That's the only diet my RO gave me. At the time of the treatment you'll likely be asked to be "gas (and fecal matter) free and full of pee" as much as possible. This helps them to be precise on targeting and the full bladder will help protect the back of your body. Apparently, radiation doesn't pass through urine all that well.

I ate my regular number of eggs, probably a 4-6 a week while on the radiation and ADT (my radiation was salvage radiation.) None of the doctors every questioned my diet as long as I was ready for treatment at the appointed time.

Best of luck to you!