Among other things, an accelerator (502) is mounted on a gantry (504) toenable the accelerator to move through a range of positions around a patient(506) on a patient support. The accelerator is configured to produce a protonor ion beam having an energy level sufficient to reach any arbitrary target inthe patient from positions within the range. The proton or ion beam passesessentially directly from the accelerator to the patient. In some examples,the synchrocyclotron has a superconducting electromagnetic structure thatgenerates a field strength of at least 6 Tesla, produces a beam of particleshaving an energy level of at least 150 MeV, has a volume no larger than 4.5cubic meters, and has a weight less than 30 Tons.
