An example particle therapy system includes a particle accelerator to output a particle beam having a spot size； a scanning system for the particle accelerator to scan the particle beam in two dimensions across at least part of a treatment area of an irradiation target； and an adaptive aperture between the scanning system and the irradiation target. The adaptive aperture includes structures that are movable relative to the irradiation target to approximate a shape to trim part of the treatment area. The part of the treatment area has a size that is based on an area of the spot size.
