Preliminary Design of a Reduced Cost Proton Therapy Facility Using a Compact, High-field Isochronous Cyclotron

It is generally agreed that, among the different kinds of radiations usable for radiotherapy, high energy proton beams exhibit the best ballistic specificity. However, the development of proton therapy has been hindered by the size, cost and complexity of high energy accelerators. We have therefore tried to design not only an accelerator, but a complete proton therapy facility where the size, the investment, the complexity and the cost of operation would be minimized. To optimize the reliability of the system we selected a non-superconducting isochronous cyclotron as accelerator. The magnet is a high field (3.09 T peak field, 2.165 T average field at extraction) deep valley design, using 190 kW in conventional coils. The complete cyclotron is split in two parts at the median plane. The upper half can be quickly raised by 1 m, using hydraulic jacks, allowing an unrestricted access to all cyclotron elements. This design feature, combined with a rapid pump downtime (30 min) should contribute towards maintaining the downtime of the accelerator to very low values. The cyclotron would then feed two or three isocentric gantries. Ion Beam Applications (IBA) has developed a new concept leading to isocentric gantries of reduced size and cost [1]. This new gantry design reaches an infinite "source to patient distance" with a gantry not exceeding 2.5 m total maximum radius. The dose delivery system has been reviewed in order to optimize dose accuracy, uniformity and speed of delivery. In the axial motion the magnetic sweep is controlled by a dose integrator, guaranteeing a uniform dose irrespective of minor intensity fluctuations.