Beam size

Beam sizes should be chosen so that the resulting high-dose volume encompasses the PTV with a minimum dose not <95% of the isocentre dose. To achieve this, field dimensions must be larger than the PTV due to the fall off in dose towards the edge of the beam. The margins required are typically in the region of 6-10 mm and depend on how many beams contribute to the fall off in dose.

A development arising from three-dimensional treatment planning that assists greatly in the determination of beam size is the beam's eye view (BEV) approach. Here the observer is placed at the radiation source and the projection of any structure that has been outlined on a CT image set is displayed on a plane normal to the central axis of the beam. A projection of the beam portal is superimposed and its size can be determined according to the projection of the PTV. The approach also assists in the selection of the optimal beam directions with respect to the separation of the target and vulnerable organs.

The technique of conformal radiotherapy is increasingly used in which the beam shape is matched more closely to the PTV. This is achieved by the use of customized blocks or multi-leaf collimators (MLC) on individual beams, and in both cases the use of BEV is essential.

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