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An important method in cancer treatment is the use of high energetic radiation. To kill tumor cells, the patient is exposed to radiation that is delivered by a linear accelerator whose beam head can be rotated about the treatment couch. Inevitably, the healthy tissue surrounding the tumor is also exposed to some radiation. So the problem arises to arrange the treatment such that the tumor receives a sufficiently high uniform dose while the damage to the normal tissue is as small as possible. The standard approach to this problem is as follows. First the patient body is discretized into the so-called voxels. The set of voxels is then partitioned into three sets: the clinical target volume, the critical structures, and the remaining tissue. There are certain dose constraints for each of these parts. Basically, the dose in the target volume has to be sufficient to kill the cancerous cells and the dose in the critical structures must not destroy the functionality of the corresponding organs. The determination of a combination of radiation fields is usually done by inverse methods based on certain physical models of how the radiation passes through a body. In the early 1990s, the method of intensity modulated radiation therapy (IMRT) was developed to obtain additional flexibility. Using a multileaf collimator (MLC) it is possible to form homogeneous fields of different shapes. By superimposing some homogeneous fields an intensity modulated field is delivered. An MLC consists of two banks of metal leaves that block the radiation and can be shifted to form irregularly shaped beams. |
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