Excluding electron beams,which,as noted,have limitations for treatment of deep tumors,protons are presently the primary beams used in particle-beam therapy.A number of cyclotrons,originally operated as nuclear research facilities,have since been converted for use as medical equipments treatment facilities.A conventional cyclotron i.e.,one that uses a fixed radio frequency (RF) accelerating voltage and a single large conventional magnet is typically limited in proton beam energy to less than 80 MeV,which limits the penetration depth to less than a few cm in tissue.
Hence,these facilities (about one-third of all proton-beam therapy facilities) are generally limited to treatment of shallow tumors,especially eye tumors,or for treatment of certain noncancerous conditions such as age-induced macular degeneration (AMD) (10,12,48,49).As many of these facilities are located in a nuclear research laboratory rather than in or near a hospital,certain treatments requiring fractionated doses or treatment done in combination with other modalities can be problematic.

Treatment of deep-seated tumors requires a proton beam energy of 200 MeV or more.At this energy,the protons’ relativistic increase of mass with increasing velocity requires the use of a large separated-sector cyclotron or high field cyclotron,such as those in use at Indiana University,Massachusetts General Hospital,and elsewhere,or a "race track" synchrotron adapted from high energy physics.An example of the latter is the synchrotron at the Loma Linda proton-beam treatment facility.Synchrotrons are also generally used to produce heavier particles (e.g.,12C ions used for radiation treatmentlead suits for radiation).
All types of proton facilities,owing to the high "magnetic- rigidity" of high energy protons,require a set of large (and costly) beam-switching magnets and patient-treatment gantries.Likewise,raster-scanning the beam and varying the dose-depth required to treat a particular tumor is not trivial,which can be done with electronic elements (active scanning) or with shaped absorbers (passive modulation).