A charged particle therapy system comprising: a particle accelerator configured to generate protons or heavy ions; a delivery device (14, 14A, 14B, 14C) including a multi-leaf collimator, "MLC", (28A, 28B); a beam path for transporting protons or heavy ions generated by the particle accelerator to the delivery device (14, 14A, 14B, 14C); and a treatment control system configured to control the operation of the particle treatment system; wherein the delivery device (14, 14A, 14B, 14C) is rotatable around a target in a subject; wherein said control system is configured to: (i) modulate, by the MLC (28A, 28B), a shape and/or size of a beam of the protons or heavy ions during rotation of the delivery device (14, 14A, 14B, 14C); and (ii) rotate the delivery device (14, 14A, 14B, 14C) about the target during delivery of at least a portion of the protons or heavy ions; characterized in that: the charged particle therapy system is further configured so that an energy, an intensity, a beam direction, and a beam shape of the protons or heavy ions are concurrently modulated during the rotation of the delivery device (14, 14A, 14B, 14C); wherein the materials and/or thickness of the MLC leaves are chosen such that the energy of the particle beam passing through the MLC (28A, 28B) is modifiable so that the MLC (28A, 28B) is configured to perform the functions of both beam shaping and beam energy modulation; and wherein the MLC (28A, 28B) is configured to shape and modulate the energy of the protons or heavy ions concurrently. a particle accelerator configured to generate protons or heavy ions; a delivery device (14, 14A, 14B, 14C) including a multi-leaf collimator, "MLC", (28A, 28B); a beam path for transporting protons or heavy ions generated by the particle accelerator to the delivery device (14, 14A, 14B, 14C); and a treatment control system configured to control the operation of the particle treatment system; wherein the delivery device (14, 14A, 14B, 14C) is rotatable around a target in a subject; wherein said control system is configured to: (i) modulate, by the MLC (28A, 28B), a shape and/or size of a beam of the protons or heavy ions during rotation of the delivery device (14, 14A, 14B, 14C); and (ii) rotate the delivery device (14, 14A, 14B, 14C) about the target during delivery of at least a portion of the protons or heavy ions; (i) modulate, by the MLC (28A, 28B), a shape and/or size of a beam of the protons or heavy ions during rotation of the delivery device (14, 14A, 14B, 14C); and (ii) rotate the delivery device (14, 14A, 14B, 14C) about the target during delivery of at least a portion of the protons or heavy ions; characterized in that: the charged particle therapy system is further configured so that an energy, an intensity, a beam direction, and a beam shape of the protons or heavy ions are concurrently modulated during the rotation of the delivery device (14, 14A, 14B, 14C); wherein the materials and/or thickness of the MLC leaves are chosen such that the energy of the particle beam passing through the MLC (28A, 28B) is modifiable so that the MLC (28A, 28B) is configured to perform the functions of both beam shaping and beam energy modulation; and wherein the MLC (28A, 28B) is configured to shape and modulate the energy of the protons or heavy ions concurrently. the charged particle therapy system is further configured so that an energy, an intensity, a beam direction, and a beam shape of the protons or heavy ions are concurrently modulated during the rotation of the delivery device (14, 14A, 14B, 14C); wherein the materials and/or thickness of the MLC leaves are chosen such that the energy of the particle beam passing through the MLC (28A, 28B) is modifiable so that the MLC (28A, 28B) is configured to perform the functions of both beam shaping and beam energy modulation; and wherein the MLC (28A, 28B) is configured to shape and modulate the energy of the protons or heavy ions concurrently.