Electron Beams

Authored by: David Thwaites , Alan McKenzie

Handbook of Radiotherapy Physics

Print publication date:  June  2007
Online publication date:  June  2007

Print ISBN: 9780750308601
eBook ISBN: 9781420012026
Adobe ISBN:

10.1201/9781420012026.ch24

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Abstract

The electron beams most commonly available in radiotherapy departments have energies of between 4 MeV and 25 MeV as produced by standard clinical linear accelerators, although some microtrons provide higher energies. Electron beams have advantages for a variety of clinical situations due to the characteristics of their depth-dose curves (Figure 24.1). They deliver acceptably uniform doses to a relatively well-defined region extending from the surface to the therapeutic range (taken as the distal 85% or 90%), which can be altered to fit the clinical situation by varying the beam energy, followed by a steep fall in relative dose that spares underlying structures. The general approach to electron-beam measurement and dosimetry is similar to that for megavoltage x-ray beams. However, there are a number of significant differences:

Electrons, being charged particles, lose energy continuously as they pass through matter. Therefore, the mean energy decreases continuously with depth in the phantom.

Electrons are light particles and undergo significant scatter. Therefore, the angular distribution of an electron beam also changes with its passage through matter. Scatter effects cause large perturbations around patient surface irregularities and inhomogeneities. Beam characteristics change when scatter equilibrium is lost. Scatter from collimators and other structures in the accelerator head has a marked effect on the detail of the dose distribution in the region around the surface and extending at least to d max and also on the link between dose and monitor units(MU). As a consequence, some of the detail of beam properties can depend much more strongly on the design of the machine head than is the case for megavoltage x-ray beams.

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