Design of a superconducting rotating gantry for heavy-ion therapy

A superconducting rotating gantry for heavy-ion therapy is being designed. This isocentric rotating gantry can transport heavy ions with the maximum energy of $430\mathrm{MeV}/u$ to an isocenter with irradiation angles of over 0–360 degrees, and is further capable of performing three-dimensional raster-scanning irradiation. The combined-function superconducting magnets will be employed for the rotating gantry. The superconducting magnets with optimized beam optics allow a compact gantry design with a large scan size at the isocenter; the length and the radius of the gantry will be approximately 13 and 5.5 m, respectively, which are comparable to those for the existing proton gantries. Furthermore, the maximum scan size at the isocenter is calculated to be as large as approximately 200 mm square for heavy-ion beams at the maximum energy of $430\mathrm{MeV}/u$. Based on the design of the beam optics, specifications of the superconducting magnets were determined. The superconducting magnets and magnetic-field distributions are designed using a three-dimensional field solver. With the calculated magnetic fields, beam-tracking simulations were performed to verify the design of the superconducting magnets, and concurrently to evaluate the field quality. With calculated beam profiles at the isocenter, we found that the positions of beam spots as well as their size and shape could be well reproduced as designed, proving validity of our design.

Figure 1

Three-dimensional image of the superconducting rotating gantry for heavy-ion therapy.

Figure 2

Layout of the superconducting rotating gantry. The gantry consists of ten superconducting magnets (BM1-10), a pair of the scanning magnets (SCM-X and SCM-Y), and three pairs of beam profile-monitor and steering magnets (STR1-3 and PRN1-3).

Figure 3

(a) Beta and (b) dispersion functions of the beam line in the rotating gantry. The blue and red lines show those for the horizontal and vertical coordinates, respectively.

Figure 4

Blue and red lines show the horizontal and vertical beam envelope functions, respectively. The solid and dashes lines represent betatron and momentum envelopes, respectively. In the calculations, typical emittances of ${\epsilon }_H={\epsilon }_V=2\pi \mathrm{mm}\mathrm{mrad}$ and momentum spread of $\Delta p/p=\pm 5\times {10}^{-4}$ for beams having $430\mathrm{MeV}/u$ were used.

Figure 5

(a) Horizontal and (b) vertical beam envelope functions with kicks of the scanning magnets, SCM-X and SCM-Y. The maximum kick angles of $\pm 18$ and $\pm 21\mathrm{mrad}$ for beams having $430\mathrm{MeV}/u$ were applied for SCM-X and SCM-Y, respectively. In the calculations, typical values of matched emittances, ${\epsilon }_H={\epsilon }_V=2\pi \mathrm{mm}\mathrm{mrad}$, and momentum spread, $\Delta p/p=\pm 5\times {10}^{-4}$, for beams having $430\mathrm{MeV}/u$ were assumed.

Figure 6

Cross-sectional view of the superconducting magnets for BM1-BM6.

Figure 7

Profile of the superconducting coil for the magnets of BM1-6 (a) before and (b) after a correction. The coil consisted of the 8 layers of the quadrupole coils and 26 layers of dipole coils. The dipole and quadrupole coils were electrically isolated, and can be independently excited.

Figure 8

Three-dimensional image of magnetic-flux density for the small-aperture magnet, as calculated by the three-dimensional electromagnetic field solver, Opera-3d. The unit of the magnetic field, $|B|$, is T. In the calculation, coil currents of 136 and 130 A were applied for the dipole and quadrupole coils, respectively.

Figure 9

Uniformity of the BL products, $\Delta B_{z}L/B_{z}L$, of the dipole field for the small-aperture magnet (a) before and (b) after corrections.

Figure 10

Uniformity of the (a) horizontal GL, $\Delta G_{x}L/G_{x}L$, and (b) vertical GL, $\Delta G_{z}L/G_{z}L$, products along beam trajectory for the small-aperture magnet before corrections.

Figure 11

Uniformity of the (a) horizontal GL, $\Delta G_{x}L/G_{x}L$, and (b) vertical GL, $\Delta G_{z}L/G_{z}L$, products along the beam trajectory, for the small-aperture magnet after corrections.

Figure 12

Uniformity of BL products, $\Delta B_{z}L/B_{z}L$, of the dipole field for various coil currents of the dipole coil along (a) the $X$ axis and (b) the $Z$ axis, as determined by the three-dimensional field calculations.

Figure 13

Profile of the superconducting coil for the magnets of BM9 and BM10 (a) before and (b) after corrections. The coil consists of 2 layers for the quadrupole coils and 22 layers for the dipole coils. The dipole and quadrupole coils were electrically isolated, and can be independently excited.

Figure 14

Three-dimensional image of magnetic-flux density for the large-aperture magnet, as calculated by the three-dimensional electromagnetic field solver, Opera-3d. The unit of the magnetic field, $|B|$, is Tesla. In the calculation, the coil currents of 231 and 200 A were applied for the dipole and quadrupole coils, respectively.

Figure 15

Uniformity of the BL products, $\Delta B_{z}L/B_{z}L$, of the dipole field for the large-aperture magnet (a) before and (b) after corrections.

Figure 16

Uniformity of the (a) horizontal GL, $\Delta G_{x}L/G_{x}L$, and (b) vertical GL, $\Delta G_{z}L/G_{z}L$, products along beam trajectory for the large-aperture magnet after corrections.

Figure 17

Uniformity of BL products, $\Delta B_{z}L/B_{z}L$, of the dipole field for various coil currents of the dipole coil along (a) the $X$ axis and (b) the $Z$ axis, as determined by the three-dimensional field calculations.

Figure 18

Beam profile at the isocenter, as calculated with tracking simulations. Each of the beam spots consisted of 5000 particles, and the 81 spots correspond to the beam with the maximum kicks of SCM-X and SCM-Y to be $\pm 18$ and $\pm 21\mathrm{mrad}$, respectively, as multiplied by 0, 0.25, 0.75, or 1.0. In the simulations, the field map for the coil currents of 231.2 A was used, and the corresponding beam energy is $E=430.96\mathrm{MeV}/u$.

Figure 19

A similar plot of the beam profile as shown in Fig. 18, but a field map for the coil currents of 91.34 A was used in the tracking simulation.

Figure 20

A similar plot of the beam profile as shown in Fig. 18, but positions of each beam spots were corrected by adjusting kick angles of the scanning magnets.