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Basics of IMRT
The basic tenet of intensity modulated radiotherapy is that the treatment beams have a non-uniform beam
intensity across their profile. Conformal radiotherapy aims to reduce normal tissue dose by shaping the beams
to the projection of the tumour, but IMRT goes a step further, enabling much more complex 3D dose distributions
to be created by the use of intensity modulated beams.
There are two ways of creating an intensity modulated beam. The first is called Dynamic IMRT, and each beam's intensity map
is created by switching on the treatment beam and then moving the beam-collimating (blocking) jaws at varying speeds across the beam profile
to build up areas of high and low intensity. Step-and-shoot (or static) IMRT involves building up each "beam" out of a series of smaller, less
intense beams to build up the desired intensity map. I guess the analogy is building a series of different height buildings across a cityscape.
With dynamic IMRT, we move from west to east across the city, building each skyscraper as we go. With step-and-shoot IMRT, we build all the bases
of the building first, then the first floors, then the second floors, and so on.
MU stands for Monitor Units, a unit of dose on the treatment machine, and is porportional to time
To treat the intensity profile on the left, the profile is divided and moved as shown so that MU/time increases with position.
The final frame shows the collimating jaw positions sliding across the field from left to right as time increases, treating the required intensity profile 
The same intensity profile is treated here with static fields, in numerical order.
While the collimating jaws move, the beam is switched off, and treatment only occurs when the jaws are stationary
OK, I've mentioned collimating jaws, but these can only control the intensity in one-dimension (as in the example above). In practice, we don't use jaws,
but Multi-Leaf Collimators (MLCs):
Siemens 27-pair MLC banks Rather than having one solid jaw that moves across the whole width of the field, each jaw is divided into a series of independently-moveable
leaves. The leaves are usually 1cm wide (projected to the isocentre of radiation - they're about half that width in physical size), although the newer Varian
machines have 0.5cm leaves, and several independent manufacturers market MicroMLCs with leaf-widths down to 3mm. By using the MLC leaves it's therefore possible
to create fully 2-dimensional intensity profiles in the plane of the beam.
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