Moving floor for radiotherapy system with cantilever gantry assembly

A proton beam therapy system with a cantilever gantry system and a moving floor system. The moving floor system includes a flexible moving floor and a track that is disposed vertically and affixed to an external wall. The track includes a lateral portion providing a linear lateral path for the part of the moving floor and non-lateral portions providing a vertical path for the excess of the floor. The non-lateral portions may be located above or below the lateral portion. During rotation of the cantilever gantry and when the beam nozzle is positioned under the patient table, an opening is maintained in the floor through which the beam nozzle protrudes. The opening follows the changing positions of the beam nozzle and moves in the floor plane in synchronization with the rotation of the gantry.