The Dynamic Multileaf Collimator

Since Jan. 15, 1999 our Clinacs are able to deliver Intensity Modulated Radiation Therapy (IMRT) with the aid of the Dynamic MLC (DMLC), and verified by VARiS 1.4d. We cannot plan clinical treatments yet, since the planning system is still to come. But we (the physicists) are not unhappy with the situation because we have some time left to think about verification of IMRT (and to play around, see below). So I hope that when the time comes and the planning system is ready, we will have solved our dosimetric problems and established acceptable QA programs concerning the DMLC.

The DMLC includes the following modes:

  1. Dose Dynamic: This is Intensity Modulation in its most general form. During dose dynamic treatments, the Gantry is at a fixed position and the MLC leaves move continuously during beam-on to produce an intensity-modulated field. Interdigitation of leaves is no problem. Dose rate is either the ceiling dose rate chosen by the operator or, if one of the leaves reaches its maximum speed, dose rate is reduced by dropping beam pulses to guarantee the desired dose distribution. The beam characteristics (dose rate vs. time, symmetry etc) were already checked with our fast measurement device from PTW and will be presented here soon). The leaf movement is described by a DMLC field file (ASCII), which consists of a list of dose fractions (between 0.0000 and 1.0000) and corresponding MLC shapes. This pair of objects defines a segment. The MLC is morphing linearly between successive segment shapes during treatment. The user can set a tolerance to a particular DMLC shape, which gives the allowed deviation of each leaf from the planned trajectory.
  2. Move and Shoot Technique: This is just a special case of Dose Dynamic Treatments. It is the sort of IMRT Siemens performs. But since the Varian DMLC is IMRT in its most general form, you can also do it on a Clinac if you want. Beam-only segments (MLC shape 2 = MLC shape 1) are followed by motion-only segments (dose fraction 3 = dose fraction 2), followed by another beam-only segment and so on.
  3. Arc Dynamic: This is nothing new and can also be planned with the Cadplan planning system. It is an Arc treatment with continuously moving leaves (linear interpolation between shapes during treatment), but dose rate and Gantry rotation speed of the Clinac is constant.

All field files are simple ASCII files that can be edited with every text editor. But security is high: a CRC code (Cyclic Redundancy Check, sort of complicated checksum) at the end of the file secures the file handling process.

All this sounds very complicated. So I probably have to point out that when you do a DMLC treatment, you hardly notice any difference between a static MLC treatment and an intensity modulated treatment with the DMLC (except that the MLC window is blue). Neither do you have to deal with files, nor do you have to care about the kinematics of the leaves. It looks very easy.

Example: The Absolut DMLC

And this is how it works: six shapes, corresponding to the dose fraction 0.000, 0.300, 0.500, 0.500, 0.700 and 1.000 have to be specified. Note that two shapes have the same dose fraction (0.500). That means that the MLC moves between these two shapes without beam output as fast as possible (move-only segment). The interpolation between the other shapes is done automatically by the MLC controller, the user needs not care about that. I captured screen shots every 0.020, so there are 52 screen shots in the animation.


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Last modified: Feb.19 1999HK