I think the range of radiation dose might vary too much to make a radiation source a totally isolated system, but trying to keep it as a simple physical lockout, I could imagine part of the start up process involving inserting a small module containing a fuse that breaks at a certain current that could be swapped out for different therapies or something. Could even add a simple spring+electromagnet mechanism that kicks that module out when power gets cut so radiotechs have to at least acknowledge the fuse before start up each time.

I will say that me pretending to know how to best design medical equipment as a web developer is pretty full of myself haha. Highly doubt whatever I'm spouting is a new idea. The idea of working on this sort of high-reliability + high-recoverability systems seems really interesting though!