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  1. Sliding Window - the MLCs are allowed to move during beam-on time and each MLC has an independent speed and position.  The dose distribution is formed by considering opposed leaf pairs. Consider a 2D plane under the MLC, for every point on the plane the amount of time it is exposed to the opening between the pair of leaves governs the dose the point receives. So by changing the time of exposure to an open beam we change the dose point by point. This is possible by varying the speed of the leaves relative to each other.
  2. Dynamic Sliding Window (Volumetric Modulated Arc Therapy, VMAT) - this is, in effect, sliding window except that the gantry is now allowed to rotate during beam-on time as well as the MLCs.
  3. Tomography (Tomotherapy) - here, we have a CT machine with 6 MV photon capabilities.  We treat slice-by-slice as the patient is indexed through the unit.
  4. Solid Brass Compensators - these patient-specific custom compensators can be milled upon demand to provide a compensator driven intensity modulation for an IMRT field.  This method is conceptually simple and does not require MLCs; however, the facility relying on this method must continually submit orders for new patient-specific compensators.

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