The characteristics of electrons were examined for five electron beam energy 6, 9, 12, 16 and 20MeV of linear accelerators(CL 1800, Varian Co) with 4(4, 6(6, 10(10, 15(15, 20(20 and 25(25 cm* field size. A commercial water scanning system
(WP600C,
Wellhofer, Inc.) with 0.1 cc ion chamber(lC 10) was used for the central-axis measurement, beam profile, iso-ionization distributions, surface dose and build-up. We calculated the R50, Rp1 E0 and Ep0 from these measurement.
The output factors were measured with 0.6cc ion chamber(NE 2571) and normalized by reference field size 15*15cm*. The energy loss rate of most electrons were roughly 2 MeV/cm in water or soft tissue. The skin-sparing effect with the all electron
beams
were modest or nonexistent. The most probable energy and mean incident energy were less than nominal energy because the intrinsic electron beam passes through the exit window scattering foil, monitor chamber, air, and other materials in reaching
the
phantom surface. The electron energy spread and angular spread are significantly increased.
In clinical practice, therefore, it is not sufficient to specify just beam energy. Isodose distributions for an individual machine, cone, and/or field size are required.
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