Having received EU clearance only three months ago, Mevion Medical Systems just won FDA approval to market its MEVION S250 proton therapy system in the U.S. Mevion promotes its system as having a smaller footprint than competing systems, the diameter of the accelerator being only 6 feet (1.8 m).
The TriNiobium Core uses TriNiobium Tin to accelerate protons to energies up to 250 MeV, allowing them to penetrate and stop an targets at up to 13 inches (32 cm) below the surface. 
Proton therapy has the great advantage of sparing tissue away from the target by depositing nearly all the energy of the protons at a specific area in the body, but it does indeed cost considerably more than traditional beam radiation.
From Mevion’s announcement:
The broad adoption of proton therapy has been greatly limited by the enormous cost, large footprint, and technical complexity of traditional proton therapy systems. Powered by a patented TriNiobium Core™, the MEVION S250 proton therapy system redefines the economics, availability, and the future direction of proton therapy, by bringing the management and operation of proton therapy to levels similar to modern X-ray radiation therapy devices.
The broad adoption of proton therapy has been greatly limited by the enormous cost, large footprint, and technical complexity of traditional proton therapy systems. Powered by a patented TriNiobium Core™, the MEVION S250 proton therapy system redefines the economics, availability, and the future direction of proton therapy, by bringing the management and operation of proton therapy to levels similar to modern X-ray radiation therapy devices.
Press release: Mevion Medical Systems Announces FDA 510(k) Clearance for the MEVION S250 Proton Therapy System
Product page: MEVION S250 Proton Therapy System
Flashback: MEVION S250 Proton Therapy Cleared in Europe