RADIOTHERAPY GEOFFREY NYACHAE HENRY

 ■ Conventional treatment uses single or opposing beams, with or without 2D dose distributions, with compensators and simple shielding.  ■ Conformal treatment involves target volume delineation of tumour and normal organs according to ICRU principles with 3D dose calculations using MLC to shape beams. ■ Complex treatment includes the use of IMRT to shape the fluence of the beam, dynamic treatments, IGRT and 3D or 4D delivery.  Most dose computation is done using 3D computerised treatment planning systems which are programmed with beam data from therapy machines. These systems require careful quality control programmes. Following production of a satisfactory isodose distribution to a given target volume using 2D or 3D algorithms, the calculations are checked by a physicist and detailed instructions for delivery are prepared by radiographers on the therapy unit.

 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.

 This is the volume of tissue that is irradiated to a dose considered significant in terms of normal tissue tolerance, and is dependent on the treatment technique used. The size of the irradiated volume relative to the treated volume (and integral dose) may increase with increasing numbers of beams, but both volumes can be reduced by beam shaping and conformal therapy.

 This is the ratio of PTV to the treated volume, and indicates how well the PTV is covered by the treatment while minimising dose to normal tissues.

 This is the volume of tissue that is planned to receive a specified dose and is enclosed by the isodose surface corresponding to that dose level, e.g. 95 per cent. The shape, size and position of the treated volume in relation to the PTV should be recorded to evaluate and interpret local recurrences (in field versus marginal) and complications in normal tissues, which may be outside the PTV but within the treated volume.

 During a fractionated course of radiotherapy, variations in patient position and in alignment of beams will occur both intra- and inter-fractionally, and a margin for set-up error must be incorporated into the CTV-PTV margin. Errors may be systematic or random. Systematic errors may result from incorrect data transfer from planning to dose delivery, or inaccurate placing of devices such as compensators, shields, etc. Such systematic errors can be corrected. Random errors in set-up may be operator dependent, or result from changes in patient anatomy from day to day which are impossible to correct. Accuracy of set-up may be improved with better immobilisation, attention to staff training and/or implanted opaque fiducial markers, such as gold seeds, whose position can be determined in three dimensions at planning, and checked during treatment using portal imaging or IGRT. Translational errors can thereby be reduced to 1 mm and rotational errors to 1°. Each department should measure i...

 Variations in organ motion may be small (e.g. brain), larger and predictable (e.g. respiration or cardiac pulsation), or unpredictable (e.g. rectal and bladder filling). When treating lung tumours, the displacement of the CTV caused by respiration can be dealt with in several ways: by increasing the CTV-PTV margin eccentrically to include all CTV positions during a respiratory cycle; by using suspended respiration with a technique such as the active breathing control (ABC) device; or by delivery of radiation using gating or respiratory correlated CT scanning and treatment. Protocols for minimising effects on the CTV of variations in bladder and rectal filling are described in relevant chapters. Uncertainties from organ motion can also be reduced by using fiducial markers, and published results are available for lung, prostate and breast tumours. Radio-opaque markers are inserted and imaged at localisation using CT or MRI, and at treatment verification, using portal films, electron...

 When the patient moves or internal organs change in size and shape during a fraction of treatment or between fractions (intra- or inter-fractionally), the position of the CTV may also move. Therefore, to ensure a homogeneous dose to the CTV throughout a fractionated course of irradiation, margins must be added around the CTV. These allow for physiological organ motion (internal margin) and variations in patient positioning and alignment of treatment beams (set-up margin), creating a geometric planning target volume. The planning target volume (PTV) is used in treatment planning to select appropriate beams to ensure that the prescribed dose is actually delivered to the CTV.

Clinical target volume (CTV) contains the GTV when present and/or subclinical microscopic disease that has to be eradicated to cure the tumour. CTV definition is based on histological examination of post mortem or surgical specimens assessing extent of tumour cell spread around the gross GTV, as described by Holland et al. (1985) for breast cancer. The GTV-CTV margin is also derived from biological characteristics of the tumour, local recurrence patterns and experience of the radiation oncologist. A CTV containing a primary tumour may lie in continuity with a nodal GTV/CTV to create a CTV-TN (e.g. tonsillar tumour and ipsilateral cervical nodes). When a potentially involved adjacent lymph node which may require elective irradiation lies at a distance from the primary tumour, separate CTV-T and CTV-N are used (Fig. 2.2), e.g. an anal tumour and the inguinal nodes. CTV can be denoted by the dose level prescribed, as for example, CTV-T50 for a particular CTV given 50Gy. For treatment of b...

Gross tumour volume (GTV) is the primary tumour or other tumour mass shown by clinical examination, at examination under anaesthetic (EUA) or by imaging. GTV is classified by staging systems such as TNM (UICC), AJCC or FIGO. Tumour size, site and shape may appear to vary depending on the imaging technique used and an optimal imaging method for each particular tumour site must therefore also be specified. A GTV may consist of primary tumour (GTV-T) and/or metastatic lymphadenopathy (GTV-N) or distant metastases (GTV-M). GTV always contains the highest tumour cell density and is absent after complete surgical resection.