IMAT/VMAT basics

One of the newest and most interesting external beam delivery techniques today goes by many names: intensity modulated arc therapy (IMAT), volumetric modulated arc therapy (VMAT), etc. To make things more confusing, vendors have each given this technique their own proprietary names: RapidArc (Varian), SmartArc (Philips), and VMAT^TM [didn't I already say VMAT?] (Elekta). Maybe the most general and correct name for IMAT would be cone-beam dynamic angle fluence modulated x-ray therapy (CBDAFMXT), but you might confuse that acronym with a chemotherapy drug name... In this post I'll discuss some of the basics of this arc-based form of IMRT (and just call it IMAT to keep things simple).

At its most basic IMAT is essentially conventional IMRT, but with the gantry moving in one or more rotating arcs, rather than delivering from a small number of fixed angles. This means that most of the concepts and advantages and disadvantages of IMRT apply to IMAT (detailed below). IMAT was developed (and marketed!) as a conventional linac-based alternative to helical tomotherapy and as a more conformal / lower critical structure dose and faster version of static angle IMRT.

The hierarchy of IMRT techniques.

In the figure showing the IMRT hierarchy, IMAT is on the branch of cone-beam, dynamic gantry IMRT. In order to deliver IMAT, a linac must have some of the following capabilities: gantry motion with beam on, dynamic MLC (i.e. leaf motion with beam on and gantry rotating), and variable dose rate.

Planning of IMAT is very similar to conventional IMRT. The plan is determined by inverse planning methods. The degrees of freedom are increased by considering gantry rotation speed, dose rate, number of field shapes, number of arcs, etc. For planning, arcs are usually approximated with a finite number of angles (e.g. 36). Constraints can be more tightly matched with multiple arcs at the expense of delivery time. Another important aspect in IMAT optimization is that MLC leaf speed limits the beam shape "distance" from one angle to the next, i.e. the MLC leaf positions cannot vary greatly from one angle to the next and thus beam shape "interconnectedness" must be taken in to account.

Advantages of IMAT include:

• Highly conformal target volume dose with lower dose to critical structures than IMRT or 3DCRT, as dose is spread over more angles.
• Faster delivery times and lower MU's (especially single arc IMAT) when compared with IMRT.
• Non-co-planar arcs possible.
• Comparable plans to helical tomotherapy, but performed with a conventional linac.

Disadvantages of IMAT include:

• Higher cost of hardware and software licensing relative to IMRT.
• Increased complexity of plans makes QA a poor diagnostic tool (i.e. hard to determine source of QA failures).

IMAT delivery techniques are the obvious(?) next step following IMRT. In fact, it's hard to come up with a list of concrete disadvantages of IMAT over IMRT. (Please comment if you feel otherwise.) In our clinic it's one of the few new techniques that everyone seems to have adopted with open arms.

Further reading:

• Cedric X Yu and Grace Tang, Intensity-modulated arc therapy: principles, technologies and clinical implementation, 2011 Phys. Med. Biol. 56 R31 doi:10.1088/0031-9155/56/5/R01 (open access).
• David Shepard, Clinical Implementation of Intensity Modulated Arc Therapy, presentation, 2009, http://www.medicaldosimetry.org/meetings/2009handouts/Shepard_VMAT.pdf