Spine SRS

Innholdsfortegnelse

Dose Modulation

How the Algorithm Varies the Dose Modulation



The VMAT optimization can utilize the capability of the treatment machine (or linac) to change the amount of monitor units delivered along different segments of the arc by varying either:

  • 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:

TypeDescription
No dose modulation: Constant dose rate and constant gantry speed
  • Dose rate only from the list of dose rates or

  • Any dose rate between the min and max of the dose rate list

Dose modulation: Variable dose rate and constant gantry speed
  • Dose rates only from the list of dose rates or

  • All dose rates between the min and max of the dose rate list

Dose modulation: Variable dose rate and variable gantry speed
  • All dose rates between the min and max of the dose rate list

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.

Details

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.

Art-No. 60919-40EN

Utgivelsesdato: 2019-05-22