The present invention relates to an irradiation unit for emitting a particle beam, a gantry including the irradiation unit, a squirrel cage structure located within and supported by the gantry, a frame arranged so as to be motionless, a moving floor including a plurality of elongated members forming a primary caterpillar, two moving floor guide units having a truncated circle shape, facing each other, attached to said squirrel cage structure and the motionless frame, respectively. Each of the two moving floor guide units includes at least a first rail, said the primary caterpillar being slidably arranged on the first rail. Each moving floor guide unit includes at least a second rail having a truncated circle shape and the moving floor includes at least one secondary caterpillar including elongated members flexibly connected to each other.