The tumour is displayed by the planning computer as a three-dimensional object, which then serves as the basis for the radiation plan created by the medical physicist. Generally, the tumour mass will be “attacked” from various directions within defined safety margins so that risk organs are omitted from the field of treatment. These fields are no longer rectangular, but are instead adapted using shields in accordance with the defined radiation volume. The aim of every radio-oncological treatment is to subject the tumour region to the highest possible radiation dose, while simultaneously protecting the risk organs to the greatest possible extent. The probability of being able to sterilise a tumour depends on its size and histology, and above all, on the dose of radiation applied. However the likelihood of side effects depends on the exposure of normal, healthy tissue to radiation. In recent years, it has become possible to control and modulate the intensity of the flow of photons within the field (intensity modulated radiotherapy, IMRT). As a result, the high-dose area can be better adapted to the defined tumour field to avoid irradiating risk organs.