Proton therapy machines: cyclotrons, synchrotrons and single-room systems

The first hospital-based proton centre opened at Loma Linda in 1990 with a synchrotron. Since then the industry has split into designs. Isochronous cyclotrons (IBA Proteus PLUS, Varian ProBeam) produce a continuous beam at a fixed energy that a degrader lowers for shallower targets, giving high dose rates but some neutron production and activation. Synchrotrons (Hitachi PROBEAT, ProTom Radiance 330, and the Japanese vendors) accelerate pulses to exactly the energy needed with no degrader, at lower intensity. Superconducting synchrocyclotrons small enough to mount on the gantry itself (Mevion S250i) or feed one room (IBA ProteusONE) created the single-room centre, which costs a fraction of a multi-room facility and is how most new centres are now built. Nearly all systems deliver pencil-beam scanning for intensity-modulated proton therapy, with cone-beam CT or in-room CT for image guidance, and vendors are adding proton arc delivery and ultra-high dose-rate FLASH modes. Upright positioning systems from Leo Cancer Care and P-Cure rotate the seated patient in front of a fixed beam, removing the gantry.

PTCOG lists more than a hundred centres in operation worldwide, concentrated in the United States, Japan, Europe and China; several countries have one national facility and most of the world has none. The clinical case rests on the absence of exit dose: firm for children, skull base and spine tumours, eye melanoma and re-irradiation, and still being tested in randomised trials for common adult cancers.