*4.3.3 Volumetric modulated arc therapy (VMAT)*

VMAT is a technique that enables the delivery of IMRT in one or more rotations of the linear accelerator gantry. The delivery time is shorter than static gantry methods while maintaining at least comparable dosimetric quality [60]. It is done by simultaneous modulation of the position of the multi-leaf collimator (MLC), dose rate, and gantry speed, while the gantry is rotating around the patient during treatment. The VMAT plan optimization is done on the same user interface as the fixed beam IMRT plan, which is the photon optimizer in the Eclipse treatment planning system. While individual optimal fluence for the beam intensity modulation is optimized for the fixed beam IMRT, the VMAT optimization considers the full rotation of the gantry by dividing it into 178 equally spaced control points [61]. Assuming that the radiation

**Figure 5.** *User interface of BAO in Eclipse treatment planning system.*

from each control point is delivered from a static gantry, the optimizer then generates the information of the MLC position, dose rate, and gantry speed altogether for the dose distribution calculation. The photon optimizer user interface for the optimization of IMRT in the Eclipse treatment planning system is shown in **Figure 6**.
