Dosimetry of small field electron beams.

Measurements of the central axis depth dose curves, isodose profiles and field size dependence of the output factors for small field electron beams from a Varian Clinac 18 linear accelerator have been performed. Energies of electron beams studied were 6, 9, 12, 15, and 18 MeV. Circular fields of 1, 2, 3, 5, and 8 cm diameter were obtained from cerrobend shields attached to the bottom face of a 15 x 15 cm2 electron cone applicator. Measurements were carried out in a water phantom with an ionization chamber and a silicon diode detector and in a polystyrene phantom with Kodak XV-2 film. For all energies studied, as the field size of the electron beam becomes smaller: (1) the depth of maximum dose shifts toward the surface, (2) the depth of 90% and 80% dose, which are common dose prescription depths, becomes smaller, (3) the surface dose increases, and (4) the dose fall-off region becomes more gradual. The output of the electron beam reduces significantly with reduction in electron beam field size as a consequence of lack of lateral equilibrium. Hence, for accurate treatment planning for small electron fields, it is essential that the beam characteristics be individually measured.