A system comprising: a particle accelerator configured to produce a particle beam (473; 504; 971) to treat a patient; a collimator (700; 720; 725; 760; 800) comprising a carrier (702; 708; 761; 762) that comprises openings (704; 716, 717; 798, 799) including a first opening (716; 798) and a second opening (717; 799), the carrier comprising a material that inhibits transmission of the particle beam, and the carrier being located between the particle accelerator and the patient; and a control system that is configured to: (i) control movement of the particle beam to the first opening to enable at least part of the particle beam to reach the patient, to change an energy of the particle beam while the particle beam remains stationary at the first opening, and to control movement of the particle beam from the first opening to the second opening, or (ii) control movement of the particle beam to a first part of the first opening to enable at least part of the particle beam to reach the patient, to change an energy of the particle beam while the particle beam remains stationary at the first part of the first opening, and to control movement of the particle beam from the first part of the first opening to a second part of the first opening. a particle accelerator configured to produce a particle beam (473; 504; 971) to treat a patient; a collimator (700; 720; 725; 760; 800) comprising a carrier (702; 708; 761; 762) that comprises openings (704; 716, 717; 798, 799) including a first opening (716; 798) and a second opening (717; 799), the carrier comprising a material that inhibits transmission of the particle beam, and the carrier being located between the particle accelerator and the patient; and a control system that is configured to: (i) control movement of the particle beam to the first opening to enable at least part of the particle beam to reach the patient, to change an energy of the particle beam while the particle beam remains stationary at the first opening, and to control movement of the particle beam from the first opening to the second opening, or (ii) control movement of the particle beam to a first part of the first opening to enable at least part of the particle beam to reach the patient, to change an energy of the particle beam while the particle beam remains stationary at the first part of the first opening, and to control movement of the particle beam from the first part of the first opening to a second part of the first opening. (i) control movement of the particle beam to the first opening to enable at least part of the particle beam to reach the patient, to change an energy of the particle beam while the particle beam remains stationary at the first opening, and to control movement of the particle beam from the first opening to the second opening, or (ii) control movement of the particle beam to a first part of the first opening to enable at least part of the particle beam to reach the patient, to change an energy of the particle beam while the particle beam remains stationary at the first part of the first opening, and to control movement of the particle beam from the first part of the first opening to a second part of the first opening.