School of Physics and Astrophysics

Volumetric Modulated Arc Therapy (VMAT)

VMAT is an advanced radiotherapy technique utilising a rotating beam. An innovation beyond Intensity Modulated Radiotherapy (IMRT), which relies on a static beam, VMAT offers several advantages.   

Volumetric Modulated Arc Therapy (VMAT) is a novel extension of Intensity Modulated Radiotherapy (IMRT) in which an optimized 3D dose distribution may be delivered in a single 360 degree gantry rotation or multiple sub-arcs while the X-ray beam is continuously on. This technique is faster than IMRT and imposes lower radiation dose to the patient and especially healthy tissues around the tumour.

This VMAT technique is made possible by a treatment planning algorithm that simultaneously changes three parameters (degrees of freedom) during treatment:

  1. Rotation speed of the gantry.
  2. Shape of the treatment aperture: using the movement of multileaf collimator (MLC) leaves and the MLC orientation.
  3. Delivery dose rate.

The main advantage of delivering radiotherapy beams in arcs is that the dose to tissues around the target is spread out. This prevents the delivery of high doses to critical organs and healthy tissues (see the figure below).

 

linear accelerator VMAT arc IMRT and VMAT comparison of dose plans

Left: Gantry is rotating around the patient while the beam is on; Centre: Dose rate, dose, gantry angle, gantry speed, and MLC position are checked at each control point (177 control points for the entire arc); Right: Comparison of dose plans for a Spinal treatment using IMRT and VMAT (RapidArc image gallery, Varian.com)

VMAT treatment delivery is two to eight times faster than dynamic IMRT since it can generate equivalent conformal dose distributions with fewer monitor units (MUs), and thus reduces the treatment times significantly. Shorter treatments lead to less patient discomfort and treatment delivery in one arc or two reduces the susceptibility to intra-fraction motion and provides quicker overall treatment slots.

VMAT’s radiobiological advantages include:

  • reduction of dose without compromising target coverage, and
  • better sparing of the organs at risk which reduces the risk of inducing secondary cancers.

VMAT has been shown to improve planning target volume (PTV) coverage by about 8% compared to conformal radiotherapy and by 6% compared to IMRT.  VMAT is able to deliver treatment plans equivalent to IMRT in terms of target conformity and homogeneity while exhibiting significant reductions in dose to critical structures and significantly reduced MUs and treatment times per fraction.

VMAT requires more extensive QA of machines and plans compared to conventional IMRT. This includes testing the radiation field flatness and dose distribution for different dose rates and testing the MLC positioning and speed. 

VMAT is currently commercially available and has been given different names by linac manufacturers: RapidArcTM (Varian Medical Systems, Palo Alto, CA), VMATTM (Elekta AB, Stockholm, Sweden) or SmartArcTM (Philips, Fitchburg, WI, USA).


 

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Last updated:
Thursday, 19 December, 2013 9:10 AM

http://www.physics.uwa.edu.au/2472290