A particle therapy system comprising: 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 an irradiation target to be treated; and an adaptive aperture between the scanning system and the irradiation target, the adaptive aperture comprising: a primary carriage configured to move in a first dimension relative to the irradiation target; secondary carriages coupled to structures to move the structures in a second dimension relative to the irradiation target, wherein the secondary carriages are coupled to the primary carriage to move the structures relative to the irradiation target in the first dimension; wherein the structures are movable relative to the irradiation target to approximate a shape to trim part of a treatment area of the irradiation target, the part of the treatment area having a size that is based on a size of an individual spot of the particle beam or an area comprised of multiples spots of the particle beam, the part of the treatment area being less than an entire treatment area of the irradiation target; wherein the structures are configured to move into, and out of, the part of the treatment area; and wherein the primary carriage and the secondary carriages are configured to move as the particle beam scans across the irradiation target, and to track the particle beam's movement during scanning. 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 an irradiation target to be treated; and an adaptive aperture between the scanning system and the irradiation target, the adaptive aperture comprising: a primary carriage configured to move in a first dimension relative to the irradiation target; secondary carriages coupled to structures to move the structures in a second dimension relative to the irradiation target, wherein the secondary carriages are coupled to the primary carriage to move the structures relative to the irradiation target in the first dimension; wherein the structures are movable relative to the irradiation target to approximate a shape to trim part of a treatment area of the irradiation target, the part of the treatment area having a size that is based on a size of an individual spot of the particle beam or an area comprised of multiples spots of the particle beam, the part of the treatment area being less than an entire treatment area of the irradiation target; wherein the structures are configured to move into, and out of, the part of the treatment area; and wherein the primary carriage and the secondary carriages are configured to move as the particle beam scans across the irradiation target, and to track the particle beam's movement during scanning. a primary carriage configured to move in a first dimension relative to the irradiation target; secondary carriages coupled to structures to move the structures in a second dimension relative to the irradiation target, wherein the secondary carriages are coupled to the primary carriage to move the structures relative to the irradiation target in the first dimension; wherein the structures are movable relative to the irradiation target to approximate a shape to trim part of a treatment area of the irradiation target, the part of the treatment area having a size that is based on a size of an individual spot of the particle beam or an area comprised of multiples spots of the particle beam, the part of the treatment area being less than an entire treatment area of the irradiation target; wherein the structures are configured to move into, and out of, the part of the treatment area; and wherein the primary carriage and the secondary carriages are configured to move as the particle beam scans across the irradiation target, and to track the particle beam's movement during scanning.