  We propose an innovative linear accelerating structure, particularly suited
for hadrontherapy applications. Its two main features are compactness and good
power efficiency at low beam velocities: the first is achieved through a high
working frequency and a consequent high accelerating gradient, the second is
obtained by coupling several H-mode cavities together. The structure is called
CLUSTER, which stands for "Coupled-cavity Linac USing Transverse Electric
Radial field". In order to compare the performance of this structure with other
hadrontherapy linac designs involving high frequencies, a conceptual study has
been performed for an operating frequency of 3 GHz. Moreover a proof of
principle has been obtained through RF measurements on a prototype operating at
1 GHz. An accelerator complex using a CLUSTER linac is also considered for
protontherapy purposes. The whole complex is called cyclinac and is composed of
a commercial cyclotron injecting the beam in a high-frequency linac.
