Immobilisation

 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 form is used to make a Perspex shell by vacuum moulding. The shell fits over the patient and fastens to a device on the couch with Perspex straps and pegs in at least five places. Alternatively, thermoplastic shells can be made by direct moulding of heat-softened material on the patient and these have a similar degree of accuracy

Relocatable whole body fixation systems using vacuum moulded bags of polystyrene beads on a stereotactic table top restrict movement to 3–4 mm and are used to immobilise the trunk and limbs with markings on the bag instead of on the patient’s skin. Where the patient has kyphosis, scoliosis or limitation of joint movement, extra limb pads or immobilisation devices may be required. Metallic prostheses, abdominal stomata and the batteries of pacemakers must be located and excluded from the radiation volume where possible. Details of immobilisation devices are discussed in each tumour site chapter.

Parameters of limb rests, thoracic or belly boards, foot rests and leg restraints, Perspex shells and skin tattoos should be clearly recorded to avoid transfer errors between the planning process and subsequent treatment. Gantry and couch top flexibility should be measured and couch sag avoided by using rigid radiolucent carbon fibre tables. Table tops must have fixtures for immobilisation devices and laser light systems are essential in CT, simulator and treatment units (Fig. 2.3). Protocols for bladder and rectal filling, respiration and other patient parameters must be documented at localisation, and reproduced daily during treatment to minimise uncertainties.

CT scans taken for localisation are only a single snapshot, and the CT scan should be repeated daily on several days to measure variation in organ motion and systematic set-up errors for an individual patient. These values can then be used to inform the CTV-PTV margin on an individual basis rather than using population derived margin values. This is known as adaptive radiotherapy (ART). Kilovoltage (kV), cone beam CT, and megavoltage (MV) imaging on treatment machines make it possible to obtain CT images immediately before treatment. While resolution is not as good as with diagnostic CT, the use of fiducial markers and image registration protocols enables daily online IGRT. With this technique only intra-fractional variations and the doctor’s CTV delineation error remain. 

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