Beam arrangements

Optimum beam arrangements required to treat a particular site have been widely investigated and adopted into protocols in each radiotherapy centre, with little variation from one centre to another. However, some sites in the head and neck and the chest may require individualized beam arrangements.

In general, co-planar beam arrangements are used, but the availability of fully three-dimensional treatment planning systems allows the placement of non-coplanar fields. The number of beams required is less with a high-beam energy and a small depth to the centre of the target. In the pelvis, three or four beams are used at the highest available energy, preferably 12-16 MV. Three beams are sufficient in the chest, using an energy of 6-8 MV, as at higher energies the range of the secondary electrons in lung tissue makes the calculation of the dose distribution uncertain. In the head and neck region, two fields are generally sufficient, using an energy 4-6 MV, as it may be necessary to provide a high dose close to the entrance surface while minimizing the exit skin dose.

The central axis of the beams pass through one point (known as the isocentre) which is placed at the centre of the PTV, with the directions of the treatment beams chosen to avoid the irradiation of sensitive structures. Placing the beams uniformly around the patient is advantageous in obtaining a uniform dose distribution over the target, and the selection of beam directions where the depth of the target is kept small reduces the overall dose to the patient.

0 0

Post a comment