OREANDA-NEWS. The National Institute of Radiological Sciences (NIRS) (President: Yoshiharu Yonekura) and Toshiba Corporation (President and CEO: Masashi Muromachi) today announced completion of a light, compact, rotating gantry*1 at NIRS’ New Particle Therapy Research Facilities*2. The gantry, the first in the world to use superconducting magnets, enables 360 degree irradiation of cancerous tumors.

The size of the equipment had been an issue in heavy-ion radiotherapy, since the accelerator and the rotating gantry, the major components that transport and adjust the carbon ion beam to generate high magnetic fields with large normal-conducting magnets. Adoption of superconducting magnets has achieved a more compact, lighter rotating gantry than the typical 25-meter long heavy-ion radiotherapy gantry; the gantry of the NIRS’ system, with superconducting magnets attached to a cylindrical rotary body, 11m in diameter and 13m long (see Figure 1).

The new gantry can rotate the radiation port in a 360-degree circle and irradiate a patient from all directions, eliminating the need to tilt the treatment couch. The gantry can also deliver a concentrated dose of radiation by irradiating targeted tumors from multi-directions, since the beam’s positioning and angle can be precisely controlled to avoid critical organs such as the spinal cord and nerves. The new gantry is patient-friendly in that it is expected to reduce not only patient stress during treatment but also the possibility of difficulties and side-effects caused by the treatment.

NIRS presented the development of the rotating gantry, together with the results of clinical trials and basic researches, at “The 2nd International Symposium on Heavy-Ion Radiotherapy and Advanced Technology”, an NIRS-sponsored symposium held at Akiba Hall in Tokyo’s Chiyoda ward on Saturday January 9. The symposium was held to promote heavy-ion radiotherapy globally, and was attended by physicians and researchers engaged in heavy-ion radiotherapy in Japan and overseas.

Development background

Heavy-ion radiotherapy uses heavy-ions that treats cancers by accelerating charged nuclei that are heavier than other radiation beams, such as X-rays and proton beams. While X-ray and proton radiation therapies generally use a gantry to deliver radiation from all angles, through 360 degrees around the patient, the world’s only previous rotating gantry for heavy-ion radiotherapy was that installed at a German institution, which is extremely large at 25m long and a massive 600 tons. This led to the accepted wisdom that a rotating gantry with normal-conducting magnets would not find wide use.

About the rotating gantry for heavy-ion radiotherapy

A major feature of the newly developed rotating gantry is the use of compact cryogenics technology based on the refrigerator-directly-cooling method. This reduces the need for liquid helium to cool the superconducting coils below 4 Kelvin (K) and thereby maintain the superconducting state*3. Consequently, the rotating gantry is extremely safe and easy to handle in ordinary medical facilities.

Another important characteristic of the rotating gantry is that its superconducting magnets are tolerant to physical vibration and magnetic field changes, unlike conventional superconducting magnets. The unique structure of the superconducting magnets ensures that the gantry maintains its superconducting state even when it is stopped and rotated. Furthermore, the use of a special structure in the gantry’s superconducting wire maintains the superconducting state even when the magnetic field intensity is changed rapidly from 1 tesla (T) to 2.9T during a few minutes of treatment, ensuring stable irradiation. This tolerance to vibration and magnetic field changes is crucial for the three-dimensional (3D) scanning irradiation*4 system achieved with the rotating gantry.

Development of the new rotating gantry and its constituent superconducting magnets was made possible by combining NIRS’ experience in developing heavy-ion radiotherapy systems and beam design technology with Toshiba’s experience in developing superconducting coils and mechanical design technology.

When a patient lies down on the robotic treatment couch in the treatment room (Figure 2), the radiation port is rotated to irradiate the patient with heavy-ion beam*5 from the optimal angles for the shape and size of a tumor identified by a 3D scanning irradiation system. Both sides of the radiation port are equipped with X-ray detectors to provide X-ray fluoroscopic observation of the tumor area and its surroundings. This makes it possible to adopt a respiratory-gated scanning irradiation system*6 that controls the beam while tracking tumor movement during respiration in real time. X-ray-image-guided, respiratory-gated irradiation and 3D scanning radiation treatment are combined.

Achievements and future outlook

A heavy-ion beam can be used to irradiate from all angles. Consequently, the rotating gantry, operating with a 3D scanning irradiation system, makes it possible to deliver a more concentrated dose of radiation than ever before to a targeted tumor, while avoiding critical organs. The new gantry is expected to reduce treatment difficulties and side-effects and to increase its efficacy compared to conventional treatment.

In Figure 3, the white dot at the center of the each circle denotes a critical organ. The five peripheral circles show irradiation of the fan-shaped tumor surrounding the organ from five different angles, and the large central circle represents the total dose distribution, with the portion receiving a concentrated radiation dose highlighted in red. It demonstrates that the new gantry can focus an adequate dose of radiation on the tumor through multi-angle irradiation, while delivering a minimal dose to the critical organ.