Modulated radiotherapy treatments such as intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT, sometimes called IMAT) involve the use of moving multileaf collimator (MLC) leaves to deliver beams with a spatially varying intensity. These treatments are planned using inverse planning, where a sophisticated computerized fluence optimization routine within the radiotherapy treatment-planning system is used to produce beams capable of delivering complex dose distributions within the patient (Figure 8.1).