Treatment planning for radical radiotherapy provides a more complicated problem and is carried out with computerized treatment planning systems. The main aim is to provide a high and uniform dose to the target volume while ensuring that the dose to any vulnerable organs is kept as low as possible and within specified constraints. It is important to determine carefully the anatomy of the patient in the region to be treated and the location of the target volume within this region, so that the dose distribution can be calculated accurately. Methods used depend on whether the dose calculations are to be performed on a single two-dimensional plane through the patient or over the full three-dimensional treatment volume.
The simplest method is to determine the external contour by a mechanical or optical device and the target position from AP and lateral radiographs. Use of a treatment simulator can also provide the information required for two-dimensional planning especially if it is equipped with a computerized tomography (CT) option.
Full CT scanning is essential for three-dimensional planning and is by far the best technique, as it provides all the necessary information as well as a density map of the patient that can be used for dose calculation.
Magnetic resonance (MR) scanning can provide better diagnostic information than CT but produces geometrical distortions in the image that must be corrected. MR scanning is therefore only used in conjunction with CT planning.
In all cases it is essential during the treatment planning process that patients consistently remain in the treatment position; immobilization devices can assist in this.
The correct determination of the planning target volume (PTV) is obviously essential to the success of radiotherapy. The gross tumour volume (GTV) is that which is palpable or radiologically demonstrable, but must be surrounded by a margin to allow for microscopic spread, giving a clinical target volume (CTV).
A further margin must be applied to the CTV to allow for geometrical inaccuracies in the treatment set-up and patient/organ movement during treatment, and this defines the planning target volume. Progression from the GTV to the PTV is straightforward in two-dimensional planning. However, enlarging the CTV to the PTV, slice by slice on CT may not give correct margins in three dimensions when there is a significant difference in the size of the CTV on adjoining slices. Further problems exist in defining margins at the superior and inferior limits of the CTV due to the width of the CT slice. Problems can be overcome by the use of three-dimensional volume-growing algorithms that are available on computerized planning systems.
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