Yoshiyuki Iwata

Performance of the NIRS fast scanning system for heavy‐ion radiotherapy

PURPOSE A project to construct a new treatment facility, as an extension of the existing HIMAC facility, has been initiated for the further development of carbon-ion therapy at NIRS. This new treatment facility is equipped with a 3D irradiation system with pencil-beam scanning. The challenge of this project is to realize treatment of a moving target by scanning irradiation. To achieve fast rescanning within an acceptable irradiation time, the authors developed a fast scanning system. METHODS In order to verify the validity of the design and to demonstrate the performance of the fast scanning prior to use in the new treatment facility, a new scanning-irradiation system was developed and installed into the existing HIMAC physics-experiment course. The authors made strong efforts to develop (1) the fast scanning magnet and its power supply, (2) the high-speed control system, and (3) the beam monitoring. The performance of the system including 3D dose conformation was tested by using the carbon beam from the HIMAC accelerator. RESULTS The performance of the fast scanning system was verified by beam tests. Precision of the scanned beam position was less than +/-0.5 mm. By cooperating with the planning software, the authors verified the homogeneity of the delivered field within +/-3% for the 3D delivery. This system took only 20 s to deliver the physical dose of 1 Gy to a spherical target having a diameter of 60 mm with eight rescans. In this test, the average of the spot-staying time was considerably reduced to 154 micros, while the minimum staying time was 30 micros. CONCLUSIONS As a result of this study, the authors verified that the new scanning delivery system can produce an accurate 3D dose distribution for the target volume in combination with the planning software.

The response of a spherical tissue-equivalent proportional counter to different ions having similar linear energy transfer.

Abstract Guetersloh, S. B., Borak, T. B., Taddei, P. J., Zeitlin, C., Heilbronn, L., Miller, J., Murakami, T. and Iwata, Y. The Response of a Spherical Tissue-Equivalent Proportional Counter to Different Ions Having Similar LET. Radiat. Res. 161, 64–71 (2004). The response of a tissue-equivalent proportional counter (TEPC) to different ions having a similar linear energy transfer (LET) has been studied. Three ions, 14N, 20Ne and 28Si, were investigated using the HIMAC accelerator at the National Institute of Radiological Sciences at Chiba, Japan. The calculated linear energy transfer (LET∞) of all ions was 44 ± 2 keV/μm at the sensitive volume of the TEPC. A particle spectrometer was used to record the charge and position of each incident beam particle. This enabled reconstruction of the location of the track as it passed though the TEPC and ensured that the particle survived without fragmentation. The spectrum of energy deposition events in the TEPC could be evaluated as a function of trajectory through the TEPC. The data indicated that there are many events from particles that did not pass through the sensitive volume. The fraction of these events increased as the energy of the particle increased due to changes in the maximum energy of the δ rays. Even though the LET of the incident particles was nearly identical, the frequency-averaged lineal energy, ȳF, as well as the dose-averaged lineal energy, ȳD, varied with the velocity of the incident particle. However, both values were within 15% of LET in all cases.

Magnetization and field quality of a cosine-theta dipole magnet wound with coated conductors for rotating gantry for hadron cancer therapy

Electromagnetic field analyses were carried out to study the influence of coated-conductor magnetisation, i.e. the screening (shielding) current, on the field quality of a dipole magnet in a rotating gantry for hadron cancer therapy. The analyses were made on the cross section of a cosine-theta dipole magnet in a rotating gantry for carbon ions, which generated 2.90 T of magnetic field. The temporal profile (temporal variation) of the magnet current was determined based on the actual excitation schemes of the magnets in the rotating gantry. The experimentally determined superconducting property of a coated conductor was considered, and we calculated the temporal evolutions of the current-density distributions in all the turns of coated conductors in the magnet. From the obtained current-density distributions, we calculated the multipole components of the magnetic field and evaluated the field quality of the magnet. The deviation in the dipole component from its designed value was up to approximately 25 mT, which was approximately 1% of the designed maximum dipole component. Its variation between repeated excitations was approximately 0.03%, and it drifted approximately 0.06% in 10 s. Some compensation schemes might be required to counteract such influence of magnetisation on the dipole component. Meanwhile, the higher multipole components were small, stable, and sufficiently reproducible for a magnet in rotating gantries, i.e. |b 3| ~ 1.1 × 10−3 and |Δb 3| ~ 0.2 × 10−3 in 10 s.

Acceleration of high current fully stripped carbon ion beam by direct injection scheme

Acceleration of a 17mA, 100keV∕u C6+ ion beam has been successfully achieved with an radio frequency quadrupole (RFQ) linac by means of “direct injection scheme.” The C6+ beam produced by a laser ion source with a Nd:YAG laser was injected to the high current RFQ linac. It has been experimentally proved that the fully stripped carbon ion beam with a current more than 10mA was accelerated by the RFQ linac.

Progress of Research and Development of Fundamental Technologies for Accelerator Magnets Using Coated Conductors

A project to develop the fundamental technologies for accelerator magnets using coated conductors is in progress. A coil-dominated magnet and an iron-dominated magnet were designed, based on the conceptual design of spiral sector fixed field alternating gradient accelerator for carbon cancer therapy and that for accelerator-driven subcritical reactor, respectively. The required winding technologies were clarified through designing the magnets. The R&D of winding technologies for coils with three-dimensional shape and those with negative bend have been carried out. The influence of the magnetization of coated conductors on the field quality of magnets was studied experimentally.

Carbon-ion radiotherapy: clinical aspects and related dosimetry

The features of relativistic carbon-ion beams are attractive from the viewpoint of radiotherapy. They exhibit not only a superior physical dose distribution but also an increase in biological efficiency with depth, because energy loss of the beams increases as they penetrate the body. This paper reviews clinical aspects of carbon-beam radiotherapy using the experience at the National Institute of Radiological Sciences. The paper also outlines the dosimetry related to carbon-beam radiotherapy, including absolute dosimetry of the carbon beam, neutron measurements and radiation protection measurements.

Molecular states in neutron-rich beryllium isotopes

Excited states in 1 2 Be and 1 4 Be were studied by using an exotic beam of 1 4 Be at 75 A MeV. A new excited state in 1 2 Be was observed at 11.8 MeV. The angular-correlation analysis showed that the spin for the new level is tentatively assigned to be 0. According to the energy-spin systematics, the observed state may be a member of the rotational band of the 6 He- 6 He cluster structure. Three excited states in 1 4 Be were also found at 10.8 MeV, 11.6 MeV, and 15.5 MeV.

Design of Superconducting Magnets for a Compact Carbon Gantry

For widespread use of rotating gantries for carbon radiotherapy, we designed a new compact gantry. This new gantry consists of three combined-function superconducting magnets having a bending angle of 90°. The dipole field of the superconducting magnets is set to be Bmax = 5.02 T, corresponding to a bending radius of 1.32 m for transporting carbon ions having kinetic energy of 430 MeV/u. The superconducting magnet also has three independent superconducting quadrupole coils, which are to be wound inside the dipole coil for beam focusing. The dipole and quadrupole coils are electrically isolated in the magnet and connected to independent power supplies so that each field component can be independently excited. Having used the combined-function superconducting magnets, the size of the rotating gantry would become very compact; the length and radius are 5.1 and 4.0 m, respectively. The magnetic field distributions of the superconducting magnets were calculated with a 3-D electromagnetic field solver, Opera-3d code. With calculated fields, the superconducting coils were designed to obtain uniform field distributions. In this paper, the design of this compact rotating gantry and the superconducting magnets is presented.

Progress of Fundamental Technology R&D Toward Accelerator Magnets Using Coated Conductors in S-Innovation Program

We report the progress of an R&D project of fundamental technologies for cryocooler-cooled accelerator magnets using coated conductors funded by the Japan Science and Technology Agency under its S-Innovation Program. Its target applications include carbon cancer therapy and accelerator-driven subcritical reactor. We have been carrying out design studies of HTS magnets for spiral sector fixed-field alternating gradient accelerators to show their feasibility for the target applications and to clarify the requirements of winding technologies. A three-dimensional winding machine has been developed to fabricate a model magnet in which winding technologies required for the designed magnet are implemented. With respect to the large magnetization of coated conductors, which is one of the big concerns on their uses in accelerator magnets, the magnetic field measurements using rotating pick-up coils have been made to clarify its influence on the multipole components of the magnetic field. A method for numerical electromagnetic field analyses of coils with three-dimensional shapes has been developed to predict the influence of magnetization on the field quality of magnets.

Development of a compact superconducting rotating-gantry for heavy-ion therapy.

An isocentric superconducting rotating-gantry for heavy-ion therapy is being developed [ 1]. This rotating gantry can transport heavy ions having 430 MeV/u to an isocenter with irradiation angles of over ±180°, and is further capable of performing fast raster-scanning irradiation [ 2]. A layout of the beam-transport line for the compact rotating-gantry is presented in Fig. 1. The rotating gantry has 10 superconducting magnets (BM01-10), a pair of the scanning magnets (SCM-X and SCM-Y) and two pairs of beam profile- monitor and steering magnets (ST01-02 and PRN01-02). For BM01-BM06 and BM09-BM10, the combined-function superconducting magnets are employed. Further, these superconducting magnets are designed for fast slewing of the magnetic field to follow the multiple flattop operation of the synchrotron [ 3]. The use of the combined-function superconducting magnets with optimized beam optics allows a compact gantry design with a large scan size at the isocenter; the length and the radius of the gantry will be to be ∼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 ∼200 mm square for heavy-ion beams at the maximum energy of 430 MeV/u. All of the superconducting magnets were designed, and their magnetic fields were calculated using the Opera-3d code [ 4]. With the calculated magnetic fields, beam-tracking simulations were made. The simulation results agreed well with those of the linear beam-optics calculation, proving validity of the final design for the superconducting magnets. The five out of 10 superconducting magnets, as well as the model magnet were currently manufactured. With these magnets, rotation tests, magnetic field measurements and fast slewing tests were conducted. However, we did not observe any significant temperature increase, which may cause a quench problem. Further, results of the magnetic field measurements roughly agreed with those calculated by the Opera-3d code. The design study as well as major tests of the superconducting magnets was completed, and the construction of the superconducting rotating-gantry is in progress. The construction of the superconducting rotating-gantry will be completed at the end of FY2014, and be commissioned within FY2015. Fig. 1. Layout of the superconducting rotating-gantry. The gantry consists of 10 superconducting magnets (BM01–BM10), a pair of the scanning magnets (SCM-X and SCMY), and two pairs of beam profile-monitor and steering magnets (STR01–STR02 and PRN01–PRN02).