Abstract

INTRODUCTION
Particle therapy has been used for more than a century for both curative and palliative treatments. In the last 20 years, it has undergone impressive technological development. The feature of particle therapy is to utilize the dose concentration with Bragg peak. By irradiating the lesion location with the aligned Bragg peak, the dose concentration is focused on the lesion leading to minimal side effect on the surrounding healthy tissues. Therefore, compared with traditional radiation therapy can lead to fewer side effects and patients can expect a higher quality of life after treatment. However, as illustrated in Fig. 1, in a traditional gantry system, magnets are mounted in the gantry structure and rotated with large mechanic arm, making it too huge for many facilities to introduce. The large dimensions (8-12m diameter) and masses (100-200 tons) of the proton gantries [1] limit the rotational speed to 1 turn/min for safety measures preventing possible collisions [2]. Besides, the mechanical system in traditional gantry system restricts the available treatment angles to several preinstalled ones, which may limit treatment planning in some cases. Therefore, to realize the compactness of gantry system, as well as continuous treatment angles for a better dose distribution, we are designing a High Temperature Superconducting Spectroscopy-type    (HTSS) Gantry System.
THE HTSS GANTRY SYSTEM
The concept of a HTSS Gantry System is illustrated in Fig. 2. The key point is that treatment angles are set by controlling magnetic field, instead of rotating the whole magnet. This idea makes it possible to realize compactness and simple mechanics, because no rotating systems are required any more. Besides, beam for treatment will be transported in an arc around the patient, making it possible to realize a continuous treatment angle range. With high temperature superconducting material, the whole system is expected to be super compact.
CONCEPTUAL DESIGN
Structure of The HTSS Gantry System
The structure of HTSS Gantry System is illustrated in Fig. 3. Beam is transported in 3 steps which are guiding, bending and concentrating. Firstly, beam will be transported to planned direction in guiding area, then bended towards the treatment table in bending area and finally concentrated for treatment in treatment area. Both of magnet fields in guiding area and bending area is excited by outer coils with the help of properly adjusted pole gaps G  in each field, given by Eq. (1).
The magnetic field in treatment area is cancelled by inner coils. During a treatment, the internal magnetic field is designed to be a static one, and the treatment angle is set by controlling Injection Scanning Magnet (ISM) which is a dipole magnet installed in the HTSS Gantry System. ISM bends the beam and inject it onto specifically designed orbit in the gantry. Because the internal magnetic field keeps static during a treatment, we can calculate the particle orbits for any treatment angle with simulated internal magnetic field profile. Figure 3: Designed structure of the HTSS Gantry System. Magnets poles for guiding area and bending area are illustrated in light gray and dark gray. Treatment area is within the range of inner coils, where is magnetic field free. Injection Scanning Magnets (ISM) are installed in the HTSS Gantry System.
Internal Magnetic Field Profile
Figure 4 shows the simulated magnetic field profile in z direction, defined in Fig. 3, as a function of radius in the midplane, when the exciting current is 21 = 3.88 × 105 [A • turns] for outer coils, and 21 = −2.67 × 105 [A · turns]  for inner coils. With the result of  B 2 (r)  in Fig. 4, the transport matrix M  for single particle can be calculated with Eq. (2).
where l is the transporting length, κ the radius of orbit in the magnetic field, z the transporting direction of particles. Transport matrix for particle beam, which requires considering beam dynamics, will be designed in the next phase. Therefore, here we only calculated elements affecting single particle in the midplane. Using the transport matrix M , we can now calculate the particle orbit in the gantry system for desired treatment angles with numerical calculation.
Particle Orbit
Figure 5 shows calculated particle orbits in the midplane of the HTSS Gantry System, for treatment angle θ  from 0° (vertical treatment) to 180° (vertical treatment from the back), with a changing step of 10°, using 230 MeV protons, the maximum energy typically used in proton therapy, with the magnetic field profile  Bz(r) When the beam is transported to ISM, we need to control the magnetic field in ISM to inject beam to the gantry system, satisfying the injection data  ( r o , φ o )  for required treatment angle, at the exit of ISM, where r o is the distance from the center, and φ o is the angle between particle direction and normal direction of the exit of the ISM. With the result of particle orbits, we can now sum up and make a graph of injection data  ( r o , φ o )  for each treatment angle  θ  (Fig. 6). The balls on the curve in Fig. 6 present 0°~180°. the injection data for each treatment angle with a changing step of  10°, but the treatment angles between steps are also valid, which means the whole curve gives the injection data for continuous treatment angle in the range of 0°~180°. In this design, ISM is installed in the Guiding Area, giving a restriction  2.3m < r o < 3.7m, which is the width of the Guiding Area. And the restriction of  φ o  will be discussed in the next phase, which mainly depends on the performance of ISM.
SUMMARY
In order to realize compactness and a continuous treatment angle of gantry system for particle therapy, we are developing a High Temperature Superconducting Spectroscopy-type (HTSS) Gantry System. Here we have presented a potential possibility for a gantry with a radius of 4m to realize continuous treatment angles in a range of 180°. In the next phase, we will study on how to satisfy the injection data with ISM, and the beam dynamics in the gantry. Besides, how to deal with the beam energy changing for different depth of tumor is also an issue for next phase. Keep the current magnetic field and calculate injection data for beam energy changing. Otherwise, use the current particle orbits and change the magnetic field strength in the gantry, which needs to consider the hysteresis of Fe pole.
REFERENCE
[1] Schulze M E, 1991, Commissioning results of the LLUMG beam switchyard and gantry, Proc.14th Particle Accelerator Conf. PAC'91, pp 610-3. [2] Nicolini G, Clivio A, Cozzi L, Fogliata A and Vanetti E, 2011, On the impact of dose rate variation upon RapidArc implementation of volumetric modulated arc therapy, Med. Phys., 38 264-71.