Proton and Carbon Linacs for Hadron Therapy

Beams of 200 MeV protons and 400 MeV/u fully stripped carbon ions are used for the treatment of solid tumours seated at a maximum depth of 27 cm. More than 100’000 patients have been treated with proton beams and more than 10’000 with carbon ions. Very low proton currents - of the order of 1 nA - are enough to deliver the typical dose of 2 Gy/l in one minute. In the case of carbon ions the currents are of the order of 0.1-0.2 nA. For this reason 3 GHz linacs are well suited in spite of the small apertures and low duty cycle. The main advantage of linacs, pulsing at 200-400 Hz, is that the output energy can be continuously varied pulse-by-pulse and in 2-3 min a moving tumour target can be covered about 10 times by deposing the dose in many thousands of ‘spots’. High frequency hadron therapy linacs have been studied in the last 20 years and are now being built as hearts of proton therapy centres, while carbon ion linacs are still in the designing stage. At present the main challenges are the reduction of the footprint of compact ‘single-room’ proton machines and the power efficiency of dual proton and carbon ions ‘multi-room’ facilities.