Conceptually, plan optimization proceeds as follows. For each treatment field, a beams-eye-view of the target is used to divide the beam into beamlets. For an 80-leaf MLC with leaves that project to 1 cm at isocenter, the beamlets are 1 cm x 1 cm. In the figure, 1 field is divided into 37 beamlets. Note that beamlets that do not intersect the target are discarded.