The most important step in planning radiotherapy is defining the target i.e. volume of tissue to be irradiated. This includes the gross tumour (e.g. as visualized clinically or by imaging) together with surrounding tissues that might have microscopic invasion of tumour cells or which are known to be at risk of spread of disease. A further margin has to be allowed for uncertainties in treatment set-up; these include variations in patient positioning, internal organ movement, and tolerances of machine calibration. It is also essential to define the position of critical organs i.e. those with a lower tolerance to radiation such as the spinal cord, eyes, and kidneys. All can be drawn directly into the planning computer on a set of CT images covering the full extent of the involved area. For less sophisticated treatments, the target and critical organs are defined using anterior and lateral radiographs.
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