Conventional simulator

 For palliative treatments, a simulator may still be used to define field borders following the 50 per cent isodose line of the beam, rather than a target volume. A simulator is an isocentrically mounted diagnostic X-ray machine which can reproduce all the movements of the treatment unit and has an image intensifier for screening. The patient is prepared in the treatment position exactly as described above for CT scanning. The machine rotates around the patient on an axis centred on a fixed point, the isocentre, which is 100 cm from the focal spot and is placed at the centre of the target volume. Digital images or radiographs are used to record the field borders chosen by reference to bony landmarks. The simulator is commonly used either for palliative single field treatments of bone metastases or to define opposing anterior and posterior fields for palliative treatment to locally advanced tumour masses.

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