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2. Production of Proton Beams for Therapeutic Applications

Proton accelerators designed for therapy need to produce beams with energy great enough to reach the deepest tumors, usually considered to be in excess of 200 MeV, a beam current adequate to achieve treatment times comparable to those of conventional x-ray treatment facilities for the range of field sizes and doses normally used in radiotherapy -approximately 10 nA or more, and the capability to provide uniform doses to the range of target volumes likely to be encountered. This corresponds to a requirement of more than 5 x 1010 protons per second extracted from the accelerator and an efficient beam spreading system. Particle accelerators are normally built with either a straight or a circular arrangement of the components. In the circular, fixed orbit geometry (synchrotron), each particle traverses the same path repeatedly during the acceleration cycle, with an increasing magnetic field keeping the particles in a fixed orbit. An extraction system is used to deflect the particles outside of the accelerator. In the circular, variable radius geometry (cyclotron), groups of particles, often referred to as "bunches" are accelerated in a fixed magnetic field and spiral outward as they gain energy. In a linear accelerator (linac), each particle generally traverses the acceleration cavity only once. As a consequence, linear accelerators are long, but do not require bending magnets to keep the particles in orbit. In a linear accelerator, a large number of bunches can be accelerated in a short time span, thereby reducing the need for a large number of protons per bunch which can be limited by space charge considerations. Thus, linear accelerators can usually accelerate higher currents than circular machines. On the other hand, it is simpler to change energies with some circular machines than with linear machines. The selection ofthe best accelerator for clinical proton therapy will be strongly influenced by the weighting of the different characteristics such as initial cost for the accelerator and the facility as a whole, reliability, space availability, number oftreatment rooms, complexity and cost of operation, range of energies desired, beam delivery technique, etc.