Geometric parameters of clinical electron beams.

The influence of different components in the treatment head of a therapy accelerator on the geometric properties of the delivered electron beams have been analysed based on the theory of small angle multiple scattering. The analyses show that it is possible to define and compute the properties of an effective electron source which, when placed in vacuum at some distance from the patient, produces exactly the same electron fluence at the patient as the real beam. The effective source is fully described by three parameters: its angular spread, its radial width and the distance from the patient. A numerical example indicates that the half width of the effective source could be considerable, often as large as 10 cm or more at low energies.