  High-frequency hadron-therapy linacs have been studied for the last 20 years
and are now being built for dedicated proton-therapy centres. The main reason
for using high-frequency linacs, in spite of the small apertures and low-duty
cycle, is the fact that, for such applications, beam currents of the order of a
few nA and energies of about 200 MeV are sufficient. One of the main advantages
of linacs, pulsing at 200-400Hz, is that the output energy can be continuously
varied, pulse-by-pulse, and a moving tumour target can be covered about ten
times in 2-3 minutes by deposing the dose in many thousands of 'spots'.
Starting from the first proposal and the on-going projects related to linacs
for medical applications, a discussion of the trend of this field is presented
focussing, in particular, on the main challenges for the future, such as the
reduction of the footprint of compact 'single-room' proton machines and the
power efficiency of dual
