Reviews of Accelerator Science and Technology

There is a clear basis in physics for the clinical use of proton and carbon beams in radiation therapy, namely, the finite range of the particle beam. The range is dependent on the beam initial energy, density and atomic composition of tissues along the beam path. Beams can be designed that penetrate to the required depth and deliver a uniform biologically effective dose across the depth of interest. The yield is a superior dose distribution relative to photon beams. There is a potential clinical advantage from the high linear energy transfer (LET) characteristics of carbon beams. This is based on a lower oxygen enhancement ratio (OER) and a flatter age response function. However, due to uncertainties relating OER with relative biological effectiveness (RBE), there is no clinical evidence to date that carbon ion beams have an advantage over proton beams. We strongly support performance Phase III clinical trials of protons vs carbon ion beams designed to feature a single variable, LET. Dose fractionation would be identical in both arms and dose distribution would be similar for the sites to be tested. For sites for which the carbon beam has a demonstrated important advantage in comparative treatment planning due to the narrower penumbra would not be selected for the clinical trials.

Medical applications represent the vast majority of the uses for radiotracers. This review addresses how accelerators are employed for the production of high purity radionuclides that are used in basic biomedical research, as well as for clinical medicine both for diagnosing disease and for treatment.

The world's first hospital-based proton treatment center opened at Loma Linda University Medical Center in 1990, following two decades of development. Patients' needs were the driving force behind its conception, development, and execution; the primary needs were delivery of effective conformal doses of ionizing radiation and avoidance of normal tissue to the maximum extent possible. The facility includes a proton synchrotron and delivery system developed in collaboration with physicists and engineers at Fermi National Accelerator Laboratory and from other high-energy-physics laboratories worldwide. The system, operated and maintained by Loma Linda personnel, was designed to be safe, reliable, flexible in utilization, efficient in use, and upgradeable to meet demands of changing patient needs and advances in technology. Since the facility opened, nearly 14,000 adults and children have been treated for a wide range of cancers and other diseases. Ongoing research is expanding the applications of proton therapy, while reducing costs.

The history and technology of medical linacs are reviewed, focusing on machine requirements for radiotherapy. Configurations used in modern machines are described and operational aspects of a gantry-style linac system are illustrated with reference to the state of the art. Aspects of structure design, modeling and testing are discussed.

The requirements for an accelerator and a beam-delivery system for medical application are described (pointing out terms of heavy-particle radiotherapy), based on 15 years of experience with carbon-ion radiotherapy at HIMAC. The present heavy-particle radiotherapy and system are reviewed here. With a view to further development of carbon-ion radiotherapy, recent progress in heavy-particle radiotherapy and a new system are also described. Finally, the clinical applications are summarized.

The use of radiofrequency linacs for hadrontherapy was proposed about 20 years ago, but only recently has it been understood that the high repetition rate together with the possibility of very rapid energy variations offers an optimal solution to the present challenge of hadrontherapy: "paint" a moving tumor target in three dimensions with a pencil beam. Moreover, the fact that the energy, and thus the particle range, can be electronically adjusted implies that no absorber-based energy selection system is needed, which, in the case of cyclotron-based centers, is the cause of material activation. On the other side, a linac consumes less power than a synchrotron. The first part of this article describes the main advantages of high frequency linacs in hadrontherapy, the early design studies, and the construction and test of the first high-gradient prototype which accelerated protons. The second part illustrates some technical issues relevant to the design of copper standing wave accelerators, the present developments, and two designs of linac-based proton and carbon ion facilities. Superconductive linacs are not discussed, since nanoampere currents are sufficient for therapy. In the last two sections, a comparison with circular accelerators and an overview of future projects are presented.

Particle accelerators were initially developed to address specific scientific research goals, yet they were used for practical applications, particularly medical applications, within a few years of their invention. The cyclotron's potential for producing beams for cancer therapy and medical radioisotope production was realized with the early Lawrence cyclotrons and has continued with their more technically advanced successors — synchrocyclotrons, sector-focused cyclotrons and superconducting cyclotrons. While a variety of other accelerator technologies were developed to achieve today's high energy particles, this article will chronicle the development of one type of accelerator — the cyclotron, and its medical applications. These medical and industrial applications eventually led to the commercial manufacture of both small and large cyclotrons and facilities specifically designed for applications other than scientific research.

Since 1990, when the world's first hospital-based proton therapy center opened in Loma Linda, California, interest in dedicated proton and carbon ion therapy facilities has been growing steadily. Today, many proton therapy centers are in operation, but the number of centers offering carbon ion therapy is still very low. This difference reflects the fact that protons are well accepted by the medical community, whereas radiotherapy with carbon ions is still experimental. Furthermore, accelerators for carbon ions are larger, more complicated and more expensive than those for protons only. This article describes the accelerator performance required for hadrontherapy and how this is realized, with particular emphasis on carbon ion synchrotrons.

An overview is given of different techniques of dose delivery applied in currently operating and planned particle therapy systems. Their advantages and disadvantages will be compared and consequences of the methods for the rest of the instrumentation will be discussed. The interrelationship between beam delivery at the patient and the accelerator system is shown by means of several concrete examples. Apart from a description of several subsystems in a particle therapy facility, design rules for optimizing the reliability of an accelerator and beam delivery system will be discussed, as well as some remarks concerning how to deal with future developments.

Ion beam therapy for cancer has proven to be a successful clinical approach, affording as good a cure as surgery and a higher quality of life. However, the ion beam therapy installation is large and expensive, limiting its availability for public benefit. One of the hurdles is to make the accelerator more compact on the basis of conventional technology. Laser acceleration of ions represents a rapidly developing young field. The prevailing acceleration mechanism (known as target normal sheath acceleration, TNSA), however, shows severe limitations in some key elements. We now witness that a new regime of coherent acceleration of ions by laser (CAIL) has been studied to overcome many of these problems and accelerate protons and carbon ions to high energies with higher efficiencies. Emerging scaling laws indicate possible realization of an ion therapy facility with compact, cost-efficient lasers. Furthermore, dense particle bunches may allow the use of much higher collective fields, reducing the size of beam transport and dump systems. Though ultimate realization of a laser-driven medical facility may take many years, the field is developing fast with many conceptual innovations and technical progress.

First, a review is given of fixed field alternating gradient (FFAG) accelerators, presenting a bit of their history and basic concepts. Special attention is paid to the concept of scaling (S-FFAG) and nonscaling (NS-FFAG) FFAGs. This notation is used only in the NS-FFAG part of the article. A discussion is then provided on operating FFAGs. A presentation is made of the designs being considered for S-FFAGs. A bit more is said about the concept of the NS-FFAG and a resonance crossing problem resulting from designs of the NS-FFAGs. A beam delivery system (gantry) employing the NS-FFAG concept is presented after that and, finally, future plans and R&D requirements are put forward.

Dielectric wall accelerators, a class of induction accelerators, employ a novel insulating beam tube to impress a longitudinal electric field on a bunch of charged particles. The surface flashover characteristics of this tube may permit the attainment of accelerating gradients on the order of 100 MV/m for accelerating pulses on the order of a nanosecond in duration. A virtual traveling wave of excitation along the tube is produced at any desired speed by controlling the timing of pulse-generating modules that supply a tangential electric field to the tube wall. Because of the ability to control the speed of this virtual wave, the accelerator is capable of handling any charge-to-mass-ratio particle; hence it can be used for electrons, protons and any ion. The accelerator architectures, key technologies and development challenges will be described.

This article is the second in a two-part account of the history of the Superconducting Super Collider (SSC). The narrative starts with the move of the SSC activities to its Waxahachie site in Texas and the subsequent organization of its administrative structure. The technical design changes incorporated into the site-specific design are described together with their impact on costs. The principal activities at the SSC — technical progress, conventional construction and planning of the experimental program — are described briefly. The article then discusses the efforts to obtain international collaboration, the growth of the opposition both among the public at large and in Congress, and the final events leading to termination of the SSC. It ends with some subjective views on what went wrong with the SSC and on the prospects for construction of large scientific facilities in the US in the future.

Robert R. Wilson was the brilliant designer, builder and founding director of the Fermi National Accelerator Laboratory with its series of high-energy physics particle accelerators providing collision energies of 200, 400 and 2,000 GeV, the most powerful facilities in their class over a period of 40 years. He undertook the "impossible" and succeeded. With untrammeled courage he challenged the establishment as he bypassed many conventional practices in accelerator design, construction and cost control. With his remarkable talents he addressed a wide range of important aspects of the relationships of art and science, elegance and efficiency and physics and society. In doing so he always found ways for his pursuit of science to support his strong advocacy for human rights, international collaboration and democracy.