A method of determining the shape of a radiation beam of a radiotherapy system at a treatment position, the system comprising: a radiation source for projecting the radiation beam towards the treatment position； and a multi-leaf collimator disposed between the radiation source and the treatment position, the multi-leaf collimator comprising an array of moveable leaves that may be positioned to intersect and block parts of the radiation beam so as to define the shape of the radiation beam at the treatment position, wherein the leaves of at least one group in the array are aligned such that the planes of the leaves in that group converge at a point that is displaced laterally from the radiation source； the method comprising: for each leaf in the array that is positioned to intersect the radiation beam, determining a projected width with respect to the radiation beam, wherein the projected width is greater than the thickness of the respective leaf； and using the projected leaf width to determine the shape of the radiation beam at the treatment position.
