RADIOTHERAPY GEOFFREY NYACHAE HENRY

 When the PTV and normal organs have been defined in 3D, the optimal dose distribution for treating the tumour is sought. Consultation with a dosimetrist is vital to select the best parameters. For example, a treatment machine must be chosen according to percentage depth dose characteristics and build up depth which will vary with energy and beam size as shown in Table 2.1. These can be used to calculate doses for treatment using single fields and to learn the construction of isodose distributions using computer modelling. Other factors to be considered in the choice of machine are the effect of penumbra on beam definition, the availability of independent or multi-leaf collimators, facilities for beam modification and portal imaging.

 A CT mode attached to the simulator gantry can be used to produce images with a relatively limited resolution during the simulation process. This provides both external contouring and some normal anatomical data, such as lung and chest wall thickness, for simple inhomogeneity corrections. Images do not give detailed tumour information or accurate CT numbers. These scans are time consuming to obtain, and are therefore usually limited to the central, superior and inferior levels of the target volume.

 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.

 Using CT data, software generates images from a beam’s eye perspective, which are equivalent to conventional simulator images. External landmarks are used to define an internal isocentre for treatment set-up. The CT simulator provides maximal tumour information as well as full 3D capabilities (unlike the simulator CT facility). It is particularly useful for designing palliative treatments such as for lung and vertebral metastases, as well as for some breast treatments using tangential beams, which can be virtually simulated and then 3D planned. The ability to derive CT scans, and provide target volume definition, margin generation, and simulation all on one workstation, provides a rapid solution.

 The patient must be in a position that is comfortable and reproducible (whether supine or prone), and suitable for acquisition of images for CT scanning and treatment delivery. Immobilisation systems are widely available for every anatomical tumour site and are important in reducing systematic set-up errors. Complex stereotactic or relocatable frames (e.g. Gill–Thomas) are secured to the head by insertion into the mouth of a dental impression of the upper teeth and an occipital impression on the head frame, and are used for stereotactic radiotherapy with a reproducibility of within 1 mm or less. Perspex shells reduce movement in head and neck treatments to about 2 mm. The technician preparing the shell must have details of the tumour site to be treated, e.g. position of the patient (prone, supine, flexion or extension of neck, arm position, etc.). An impression of the relevant area (made using quick setting dental alginate or plaster of Paris) is filled with plaster and this ...

 These are critical normal tissues whose radiation sensitivity may significantly influence treatment planning and/or prescribed dose. Any movements of the organs at risk (OAR) or uncertainties of set-up may be accounted for with a margin similar to the principles for PTV, to create a planning organ at risk volume (PRV). The size of the margin may vary in different directions. Where a PTV and PRV are close or overlap, a clinical decision about relative risks of tumour relapse or normal tissue damage must be made. Shielding of parts of normal organs is possible with the use of multi-leaf collimation (MLC). Dose–volume histograms (DVHs) are used to calculate normal tissue dose distributions.