A superconducting cyclotron accelerating q/A=0.5 ions up to 250 A.MeV, for medical applications and radioisotopes production (SCENT project) is being studied at Laboratori Nazionali del Sud in Catania. The RF system, working in the fourth harmonic, is based on four cavities operating at 93 MHz, which are connected in the central region. The paper describes an unusual multistem RF design, performed with 3D electromagnetic codes. The aim is to obtain a cavity, completely housed in the valley, with a voltage distribution going from 65 kV in the injection region to a peak value of 120 kV in the extraction region, and a low power consumption.