A particle therapy system comprising: a particle accelerator to output a particle beam for application to an irradiation target; a scanning system to scan the particle beam across at least part of the irradiation target, wherein a spot of the particle beam includes a cross-sectional area of the particle beam; an adaptive aperture between the particle accelerator and the irradiation target, characterised in that: the adaptive aperture comprises: a primary carriage configured to move in a first dimension relative to the irradiation target; and secondary carriages (714, 715) coupled to the primary carriage each configured to move in a second dimension relative to the irradiation target and each comprising multiple leaves (701; 735; 736) that are extendable and retractable relative to the particle beam to produce shapes to trim a single spot or spots of the particle beam in a treatment area during scanning; and by further comprising: a control system comprising one or more processing devices configured to perform operations comprising: receiving, from a treatment planning process, information generated by iterating through different combinations of particle beam shape and adaptive aperture configurations to obtain a resulting dosage matching a target dosage; and configuring the adaptive aperture based on the information, where configuring comprises moving a position of the primary carriage, moving a position of one or more of the secondary carriages, or extending or retracting one or more of the leaves. a particle accelerator to output a particle beam for application to an irradiation target; a scanning system to scan the particle beam across at least part of the irradiation target, wherein a spot of the particle beam includes a cross-sectional area of the particle beam; an adaptive aperture between the particle accelerator and the irradiation target, characterised in that: the adaptive aperture comprises: a primary carriage configured to move in a first dimension relative to the irradiation target; and secondary carriages (714, 715) coupled to the primary carriage each configured to move in a second dimension relative to the irradiation target and each comprising multiple leaves (701; 735; 736) that are extendable and retractable relative to the particle beam to produce shapes to trim a single spot or spots of the particle beam in a treatment area during scanning; and a primary carriage configured to move in a first dimension relative to the irradiation target; and secondary carriages (714, 715) coupled to the primary carriage each configured to move in a second dimension relative to the irradiation target and each comprising multiple leaves (701; 735; 736) that are extendable and retractable relative to the particle beam to produce shapes to trim a single spot or spots of the particle beam in a treatment area during scanning; and by further comprising: a control system comprising one or more processing devices configured to perform operations comprising: receiving, from a treatment planning process, information generated by iterating through different combinations of particle beam shape and adaptive aperture configurations to obtain a resulting dosage matching a target dosage; and configuring the adaptive aperture based on the information, where configuring comprises moving a position of the primary carriage, moving a position of one or more of the secondary carriages, or extending or retracting one or more of the leaves. receiving, from a treatment planning process, information generated by iterating through different combinations of particle beam shape and adaptive aperture configurations to obtain a resulting dosage matching a target dosage; and configuring the adaptive aperture based on the information, where configuring comprises moving a position of the primary carriage, moving a position of one or more of the secondary carriages, or extending or retracting one or more of the leaves.