That's not how radiation devices are regulated.
There is an intended use statement for each radiation device. Varian keeps it nice and broad.
"The TrueBeam™ system is intended to provide stereotactic radiosurgery and precision radiotherapy for lesions, tumors, and conditions anywhere in the body where radiation treatment is indicated."
If you want to be more specific detail on how the devices should be utilized, you have to look to all the usual sources (books, journal articles, guidelines, etc). As far as the FDA is concerned, you can use a linac for anything you (a qualified physician) think it's indicated for.