Multi-leaf collimator, particle beam therapy system, and treatment planning apparatus

A multileaf collimator that is disposed on a beam orbit of an accelerated charged particle beam and that limits or forms an irradiation field of the charged particle beam in such a way that the irradiation field conforms to an irradiation subject, the multileaf collimator comprising:

• a leaf row in which a plurality of leaf plates are arranged in the thickness direction thereof in such a way that the respective one end faces of the leaf plates are trued up;
• and
• a leaf plate drive mechanism that drives each of the plurality of leaf plates in such a way that the one end face approaches or departs from a beam axis of the charged particle beam, wherein each of the plurality of leaf plates is formed of part of a ring that is created by rotating a cross section surrounded by two arcs that are coaxial with each other with respect to a first axis including a first reference point and have respective different radiuses and by two straight lines that are on a plane perpendicular to the first axis and extend from the first reference point around a second axis that is on the beam axis, includes a second reference point being apart from the first reference point, and has a direction different from that of the first axis.