Effect of incident energy

The depth of penetration of an electron beam is determined by its incident energy. Practical range (in centimetres) of an electron beam in water is given approximately by:

where E0 is incident beam energy expressed in mega electron volts (MeV).

Similarly, the clinically useful range—the depth at which the dose falls to 80% of its maximum value—is given by:

The surface dose (commonly defined as dose at 0.5 mm depth) is significantly higher for an electron beam than for a megavoltage photon beam and ranges from about 85% of dose maximum at low energies (less than 10 MeV) to about 95% at higher energies. The rate at which dose falls off beyond the depth of dose maximum is also energy-dependent, with the rate of dose fall-off decreasing as the beam energy increases.

Accelerators that offer an electron beam mode generally allow selection of one of several available electron beam energies. Beam energies may range from as low as 4 MeV up to 50 MeV in some cases. However, energies most commonly used clinically tend to be in the range 6-15 MeV.

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