The present disclosure relates to an accessory holder attachable to or integrated in the nozzle of an apparatus for particle beam irradiation treatment. The accessory may be an aperture piece, a range shifter or any other element that can be placed in the beam path between the outer end of the nozzle and the irradiated target. The accessory holder may be equipped with first displacement means for moving the accessory away from or towards the nozzle, thereby moving the accessory forwards and backwards in the direction of the beam and second displacement means for moving the accessory into or out of the beam path. Measurements or treatment steps may be performed with and without the accessory in the beam path, without interrupting the treatment.
