低LETの放射線治療(X線治療,陽子線治療)では,照射野内での線質の変化は臨床上無視できるほど小さく,治療効果は吸収線量に基づいて経験的に予測されていた。高LETである炭素線を用いた治療の場合,照射野内で特に深さとともに線質は著しく変化し,これに伴って治療効果(RBE)にも大きな変化をもたらす。このことから,従来とは違った治療モデルが不可欠となった。日本における炭素線治療の始まりについて述べ,スキャニングへの移行,microdosimetryの手法を取り入れた高度なモデルの開発,また検出器についての開発について説明する。
炭素線治療を始めるにあたって,最大の問題は治療に適した拡大Braggピークをどのようにして作るかということであった。この拡大Braggピークを設計するには重粒子線治療の本質的な問題を様々な仮定を駆使することが必要であった。治療開始から25年たった今でも未解決の問題がたくさん残っている。ここでは日本における炭素線治療の開始の際に行った,これらの問題への取り組みについて説明する。
炭素線は従来の放射線と異なり,線量のみならず線質が照射野内で劇的に変化し,これに伴って生物・臨床効果も飛程終端に向けて高まる。安全で効果的な炭素線治療の実現のためには,線質の変化を生物効果と対応付けるための生物効果モデルの確立が不可欠である。本稿では,炭素線治療の実現のため放射線医学総合研究所で開発された日本式の生物モデルの進展を中心に,ドイツモデルとの比較や,治療ビームの線量・線質を測定するために開発されてきた検出器開発の概要を俯瞰する。
近年,呼吸同期照射法,スキャニング照射法や強度変調粒子線治療などの高度な治療技術が開発され,高精度な粒子線治療が可能になってきている。これらの技術を活かしながらより高い治療効果を実現するためには,現在の臨床効果モデルでは反映してこなかった,修復や酸素効果などの放射線感受性を左右する因子をも適切に線量設計に反映すべきであろう。本稿では,それらの取り組みについて紹介する。
我々は,重粒子線治療を標準的治療法として確立すること,さらに治療成績を向上することを目的として研究を遂行してきた。そのために①重粒子線治療多施設共同臨床研究グループ(J-CROS)を設立し,比較研究を行い,治療の標準化を進め,②量子科学技術研究開発機構の技術を活用し,量子メスなど,より治療効果の高い照射法の開発に向け研究してきた。今回,我々の施行してきた臨床研究の概要と疾患別の最新の成果と展望を紹介する。
Exposure to space radiation will be a limiting factor in future missions beyond low Earth orbit, such as to Mars. Mission durations will range from many days to weeks and many months, all spent outside the geomagnetic field, exposed to chronic galactic cosmic rays (GCR) as well as periodic solar particle events (SPE). Experiments in space are difficult and expensive. While it is not feasible to replicate the full space radiation environment on the ground, some particle accelerators are capable of producing significant components of the GCR. From the late 1990s through the present day HIMAC has been one of the most important such facilities. In this section we review a number of experiments in which HIMAC has been used to further international research in space radiation physics and biology, including radiation shielding, detector development for crewed and robotic spacecraft, radiation effects on biological organisms and electronics.
Measurements of nuclear fragmentation cross sections and of particle production in thick targets are needed for the development of space radiation shielding. Cross sections are used as source terms for models of fragmentation and transport, and thick target measurements for model validation and to evaluate candidate shielding materials and concepts. Here we briefly review how HIMAC ion beams have been used to simulate elements of the space radiation field for cross section and thick target measurements.
The ICCHIBAN project was an international collaboration to intercalibrate and intercompare the response of the different detectors and instruments used for radiation dosimetry aboard manned spacecraft. The objectives of the ICCHIBAN project were: 1) to determine the response of space radiation instruments and dosimeters to heavy ions of charge and energy similar to that found in the galactic cosmic radiation (GCR) spectrum; 2) to compare the response and sensitivity of various space radiation monitoring instruments and aid in reconciling differences in measurements made by various radiation instruments during space flight; and 3) to establish and characterize a heavy ion “reference standard” against which space radiation instruments can be calibrated. ICCHIBAN experiments were carried out at a number of particle accelerator facilities, the vast majority, eight, using the HIMAC heavy ion accelerator at the National Institute for Radiological Sciences, Chiba, Japan. Benefits of the ICCHIBAN project included the identification and correction of problems in calibration and data interpretation of a number of active space radiation instruments, and the demonstration of the overall efficacy and reproducibility of passive radiation dosimeters, especially luminescence-based detectors such as TLD and OSLD used in conjunction with CR-39 PNTD.
Space radiation exposures are distinct from those in charged particle radiotherapy in that they are typically low dose, low dose rate and mixed field. It is not feasible to exactly replicate these conditions in ground-based facilities, but carefully designed experiments at HIMAC have provided insight into the biological effects of space radiation.
The mechanisms of Single Event Effects (SEEs) on Silicon Carbide (SiC) power devices are becoming clearer. However, to completely understand the mechanisms of SEEs on SiC power devices and to explore radiation hardened technologies for SiC power devices, it is necessary to study the radiation response of SiC power devices using high-energy heavy ions at accelerator facilities such as the Heavy Ion Medical Accelerator in Chiba (HIMAC). The radiation hardening test methodology using high energy heavy ions is reviewed and the application of HIMAC for SEE testing is introduced with recent results from tests of SiC power devices.
A variety of radiation detectors and instruments have been deployed to characterize the radiation environment in low and high Earth orbit, lunar orbit, Mars orbit and on the surface of Mars. Here we discuss the testing and calibration of these detectors using HIMAC ion beams.