Synchrotrons for hadron therapy: Part I

The treatment of cancer with accelerator beams has a long history with betatrons, linacs, cyclotrons and now synchrotrons being exploited for this purpose. Treatment techniques can be broadly divided into the use of spread-out beams and scanned `pencil’ beams. The Bragg-peak behaviour of hadrons makes them ideal candidates for the latter. The combination of precisely focused `pencil’ beams with controllable penetration (Bragg peak) and high, radio-biological efficiency (light ions) opens the way to treating the more awkward tumours that are radio-resistant, complex in shape and lodged against critical organs. To accelerate light ions (probably carbon) with pulse-to-pulse energy variation, a synchrotron is the natural choice. The beam scanning system is controlled via an on-line measurement of the particle flux entering the patient and, for this reason, the beam spill must be extended in time (seconds) by a slow-extraction scheme. The quality of the dose intensity profile ultimately depends on the uniformity of the beam spill. This is the greatest challenge for the synchrotron, since slow-extraction schemes are notoriously sensitive. This paper reviews the extraction techniques, describes methods for smoothing the beam spill and outlines the implications for the extraction line and beam delivery system