the size of the cyclotron and potentially leading to easier beam transport solutions and substantially less shielding and siting costs.

This reduction is a consequence of the inverse relationship between the radius of the cyclotron and the magnetic field, as shown below:

Ef ≈ Kr2B2

where Ef is energy, K is a constant, r is the cyclotron radius, and B is the magnetic field. Thus cyclotrons can be made very compact by going to high magnetic fields. Currently two superconducting cyclotrons have been built for proton radiotherapy and are treating patients on a regular basis (MSU, 1993; Miyata et al., 2012). One machine is at the Paul Scherrer Institute in Villigen, Switzerland (PROSCAN) and the other is installed at the Reinecker Proton Therapy Center in Munich, Germany. Both machines are isochronous cyclotrons built by the same company and based on a 1993 design done by the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. They are cooled with liquid helium but maintained cold by cryocoolers in a closed cryogenic system, similar to the methods used to cool MRI magnets. These machines are more compact than resistive cyclotrons, reducing the size and weight from 4.3 m and 220 metric tons down to 3.1 m diameter and 90 metric tons.

Although this is a factor of 2 reduction in weight, these machines used NbTi superconductor and limited the central gap field to 2.4 T. Newer designs by other organizations are capitalizing on the very high current density and high critical field of Nb3Sn to develop much more compact synchrocyclotrons. The most advanced of these designs is the Mevion S250 proton synchrocyclotron built by Mevion Medical Systems, Inc., based on technology licensed by MIT. The concept developed at MIT is based on using a Nb3Sn magnet generating 9 T at the pole gap with a peak field at the windings of ~11 T. This design takes advantage of a very high current density superconducting wire developed by U.S. industry using funding from the U.S. high-energy physics research program.

The device built by Mevion Medical Systems has a diameter of only 1.8 m and weighs about 20 metric tons. It is small enough and light enough to be placed on the treatment gantry so that the entire cyclotron rotates around the patient, as shown in Figure 7.9. The compact size and light weight of the cyclotron not only reduce cost but also eliminate the stationary beam transport system as well as the heavy gantry-mounted beam transport magnets, thus reducing the gantry weight as well. These systems can be installed as individual treatment machines instead of the contemporary device, which uses a single accelerator with beam line transport of protons to multiple rooms. Thus the initial capital investment in establishing a center that can scale to multiple treatment rooms is reduced by a factor of 10. This

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