A particle therapy assembly for irradiating a moving target with a particle beam is provided. The assembly comprises breath holding means for holding a patients breath during the delivery of the particle beam to the target. With the assembly and method of invention, displaying means are provided to the patient to visualize the actual position of the moving target together with a prescribed target position, so that the patient can manage his breath and actively contribute to the target positioning before actuating the breath holding means and starting the delivery of the particle beam.