Its all good, it may also depend on the type of radiation. They can do a pretty good job with proton based focal radiation, but there is still spillover. It has gotten to a point in which if they can spare the hippocampus, they can irradiate the brain repeatedly. There are some side effects where the beam passes through (hair loss, irritation, cell death, etc.) regardless of the type of treatment they're doing. Essentially they are using a proton/photon as a bowling ball, causing chaos as it passes through molecules, spinning off electrons which create free radicals that will (hopefully) blow out the DNA of the cells they run into.

There are some promising trials out there focused on getting treatment through the blood-brain barrier. In the future, maybe in just a few years, you'll start to see some spine catheter trials to directly get chemo drugs and/or antibody conjugate into the CSF. If you can just hang on, there is reason to hope.

This is wrong. There are multiple ways to utilize radiation, but all involve spillover into outside areas. Whether it's photon or proton therapy you're going to have side effects, if it's in the brain, you're going to have serious side effects.

Source: I am dying and currently undergoing brain/spine proton radiation therapy at the Mayo clinic.