Multi-leaf collimator (MLC) in an application of Intensity Modulated Radiation Therapy (IMRT). Diagram by Anders Brahme.
The dose distribution inside and around a selected target volume and in the surrounding organs at risk can be optimized further by varying the intensity distribution of the radiation inside an irradiation field. The required variation depends on the anatomy and the morphology of the target volume. With traditional treatment machines this was possible by inserting a modulating transmission filter in the beam.
With the MLC a clinical treatment beam can be split up in small beamlets and each beamlet can deliver a specific dose. This irradiation technique is called Intensity Modulated Radiation Therapy or IMRT. Each block on the grey plane in the figure represents a beamlet, the height of the block indicates the contribution to the total dose. The great advantage of the MLC in this approach is that the irradiation process can proceed automatically. In this way IMRT becomes feasible in a busy radiotherapy department.