A proton beam therapy system with a cantilever gantry. The cantilever gantry has one end portion (the fixed end portion) affixed to an external structure that supports the weight of the gantry. The remainder of the gantry is suspended and the free end portion is coupled to a beam nozzle. A main bearing is coupled to the fixed end portion and enables the gantry to rotate in a full range of 360° around the iso-center. A large counterweight can be disposed in the fixed end portion to keep the system center of mass close to the bearing. The gantry may have a monocoque housing, including a cantilever section enclosing the magnets and other components of the gantry beamline and a drum section on which the bearing is placed.
