Initial VMAT Plan
A fluence-based optimization similar to IMRT
A special purpose leaf sequencer that tries to capture the largest fluence contributions from each arc direction
The resulting arcs fulfill the PTV prescription and obey machine constraints, such as leaf speed limits. Therefore, these arcs are already deliverable by a linear accelerator. However, dosimetrically they cannot produce a high quality treatment plan yet. Instead, they are only used as a starting point for the core optimizer. Thus, this part of the optimization is relatively short compared to the second step.
Optimized VMAT Plan
The second step of the optimization is a direct aperture-based optimizer (DAO) that modifies both leaf positions and control point weights to fulfill the dosimetric requirements of the treatment planner as best as possible. Direct aperture-based means that during the optimization only deliverable treatment plans are evaluated and compared to each other. Thus, no subsequent leaf sequencing is required. Furthermore, the optimization uses a brute force approach for finding the best solution and is therefore independent of any special form of the objective function. This allows increased flexibility in formulating the optimization goals (e.g., easy integration of leaf movement limitations or monitor unit requirements). Therefore, depending on your specific requirements, various optimization goals can be set via the sliders provided in the GUI.
This part of the optimization will already use the normal dose calculation algorithm (Pencil Beam or Monte Carlo, depending on your selection). Thus, most of the optimization time is spent in this second step. However, no subsequent dose calculation is required. What you see during and at the end of the optimization is already the final treatment plan with the final dose calculation.