How the Algorithm Varies the Dose Modulation
The dose rate (monitor units per minute)
The gantry speed (gantry movement per minute)
This is in addition to the modulation caused by moving the MLC leaves.
Types of Dose Modulation
The specific linac and R&V setup determines what kind of dose modulation is used. There are three types of dose modulation:
|No dose modulation: Constant dose rate and constant gantry speed|
|Dose modulation: Variable dose rate and constant gantry speed|
|Dose modulation: Variable dose rate and variable gantry speed|
If dose rate modulation is used, there is no limit to how much the dose rate can change between control points. However, if gantry speed modulation is used, there is a constraint on how much the gantry speed can change between control points. Physics Administration allows the configuration of the capabilities of the linac.
The resulting treatment plan of a VMAT optimization does not specify the dose rate and/or gantry speed of individual control points, but rather specifies how many monitor units shall be delivered between successive control points. It is up to the linac to decide how to actually deliver the number of requested monitor units by varying the dose rate and/or the gantry speed. Nevertheless, it is important to specify the correct capabilities in Physics Administration in order to be able to produce treatment plans that can be successfully delivered by the linac. This is because certain machine limitations (e.g., the maximum gantry speed changes between control points) must be known when creating the treatment plan.
When delivering an arc, the treatment machine always tries to find (within the limits of the machine capabilities) a combination of dose rate and gantry speed that results in the fastest possible treatment of the given amount of monitor units. For machines capable of varying both the dose rate and the gantry speed this means that the machine uses either the maximum dose rate or the maximum gantry speed for each segment between two successive control points.