A beam transport assembly conveys a particle beam from a particle source to an irradiation nozzle, which rotates about a swivel axis at the horizontal input of the nozzle. A support can move horizontally in a plane perpendicular to the swivel axis. The beam transport assembly can change a path length of the particle beam so as to follow a vertical location of the swivel axis of the irradiation nozzle with respect to the support. A controller can coordinate the path length change of the particle beam, rotation of the irradiation nozzle about the swivel axis, and/or horizontal motion of the support to provide irradiation of a supported object from various angles in the plane perpendicular to the swivel axis while maintaining the irradiation nozzle at a constant distance from the supported object.