Photons, electors, and protons all have exponential drop-offs in energy as they pass through the body. (Obviously much flatter for protons or x-rays than for visible light.) If you're going to go very deep, you're going to want to use at least X-rays if not electron or proton beams. Even with UV, it's just not very efficient to lose such a high percentage of energy near the surface.

But, you're right, probably 20 years ago I saw photos of a device for converging proton beams on brain cancer. It just doesn't work very well using anything weaker than X-rays for anything very deep.