The K250 Proton-therapy Cyclotron

Design detail of the K250 that forms the basis for ACCEL corporation’s proton-therapy cyclotron. more

NSCL developed a conceptual design for a superconducting K250 cyclotron for proton-therapy of cancer patients. This conceptual design forms the basis for the new 250 MeV proton accelerator built by ACCEL corporation to become part of a proton therapy facility at the Paul Scherrer Institute, Villigen, Switzerland. Our laboratory collaborates with ACCEL and provides technology transfer and expert advice.

A beam of protons has many advantages for radiation treatment of cancer. Since protons have an electrical charge, they can be focused to a “pencil-thin” beam if desired. They also have the property that most of the beam will stop at the same depth in the patient’s body. This depth can be calculated, and the beam energy can be chosen to make most of the beam stop at the cancer, destroying the cancer in the process with minimal damage to surrounding tissue. For this reason, proton therapy is the treatment of choice for inoperable tumors, such as tumors located in the eye, close to main arteries, or in regions of the brain that are difficult to access by surgery.

The superconducting proton cyclotron has many design features in common with NSCL cyclotrons. In the model drawn you can see a section of the cylindrical yoke of the main magnet drawn in green in the diagram, with the north and south magnetic poles colored yellow. The superconducting coils are wound around the magnetic pole tips, and the coils are immersed in liquid helium (the cylinder that has been cut away in the drawing to show the coils inside) to keep them at the superconducting temperature of about -450 °F or 4 Kelvin. The four spiral-shaped high-voltage electrodes that accelerate the protons are shown in pink, and the high-voltage signals go in through the tuning elements on the top of the cyclotron (there are symmetric tuning elements on the bottom of the cyclotron that do not show in the drawing). In the picture, the cyclotron is open; in the operating position, the top cap is lowered to mate with the yoke.