Beam weight

The relative contribution of a beam to the treatment plan is known as the beam weight. The definition varies between centres and is defined either as the relative contribution of the dose to the isocentre or to the reference point. The adjustment of beam weights is a method of achieving uniformity of dose over the PTV and giving approximately the same dose to the isocentre from each beam will give good uniformity. Care should be taken to ensure that the entrance dose under the beam portals is not excessive.

The inherent dose distribution of a beam can be altered by the insertion of a wedge-shaped filter that produces a dose gradient in one dimension across the beam. Wedges can be used to compensate for obliquity at the skin surface or to assist in obtaining dose uniformity in the PTV when the beams are not uniformly spaced around the patient. Wedges are characterized by the angle through which the isodose lines are tilted and normally a selection are available (or can otherwise be achieved), producing tilts of up to 60°.

Selection of the optimum beam weights and wedges is assessed by the closeness of the resulting dose distribution to that required by the radiation oncologist. Modification of the inherent dose distribution over the beam in two dimensions is possible by the use of physical compensators or, more recently, by the dynamic movement of the leaves of an MLC.

The use of the treatment simulator depends on the patient data used for planning. In three-dimensional CT planning, the radiation oncologist usually outlines the CTV on the relevant CT slices together with the outline of any vulnerable organs. The CT data represent the patient with computer graphics used to simulate the treatment beams. This can be thought of as virtual simulation and the patient only visits the treatment simulator at the end of the treatment planning process to have the treatment verified.

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