Carbon/proton therapy: A novel gantry design

A major expense and design challenge in carbon/proton cancer therapy machines are the isocentric gantries. The transport elements of the carbon/proton gantry are presently made of standard conducting dipoles. Because of their large weight, of the order of $\sim 100\mathrm{\text{tons}}$, the total weight of the gantry with support structure is $\sim 600\mathrm{\text{tons}}$. The novel gantry design that is described here is made of fixed field superconducting magnets, thus dramatically reducing magnet size and weight compared to conventional magnets. In addition, the magnetic field is constant throughout the whole energy region required for tumor treatment. Particles make very small orbit offsets, passing through the beam line. The beam line is built of combined-function dipoles such as a nonscaling fixed field alternating gradient (NS-FFAG) structure. The very large momentum acceptance NS-FFAG comes from very strong focusing and very small dispersion. The NS-FFAG small magnets almost completely filled the beam line. They first make a quarter (or close to a quarter) of an arc bending upward and an additional half of a circle beam line finishing so that the beam is pointed towards the patient. At the end of the gantry, additional magnets with a fast response are required to allow radial scanning and to provide the required position and spot size. The fixed field combined-function magnets for the carbon gantry could be made of superconducting magnets by using low temperature superconducting cable or by using high temperature superconductors.

Figure 1

Particle orbits in the basic gantry cell for a kinetic energy range of $149–400\mathrm{MeV}/u$.

Figure 2

Betatron functions and magnets in the basic cell.

Figure 3

The nonscaling FFAG, used for the gantry design. Orbit offsets are very small.

Figure 4

Matching cell to the accelerator, the gantry, and the final focus with scanning magnets above the patient.

Figure 5

Orbit functions in the gantry at the central momentum.

Figure 6

Maximum orbit offsets dependence on ${\theta }_{BF}/{\theta }_{BD}$ at the end of the gantry.

Figure 7

Initial conditions for particles in the $(x,x^{\prime })$ and $(y,y^{\prime })$ phase space.

Figure 8

Tracking results in $(x,x^{\prime })$ phase space at the end of the nonscaling FFAG gantry.

Figure 9

Tracking results in $(x,y)$ phase space at the end of the nonscaling FFAG gantry.

Figure 10

Tracking results in $(y,y^{\prime })$ phase space at the end of the nonscaling FFAG gantry.