CHARACTERISTICS OF SCATTERED ELECTRON-BEAMS SHAPED WITH A MULTILEAF COLLIMATOR

Characteristics of dual-foil scattered electron beams shaped with a mu ltileaf collimator (MLC) (instead of an applicator system) were studie d. The electron beams, with energies between 10 and 25 MeV, were produ ced by a racetrack microtron using a dual-foil scattering system. For a range of field sizes, depth dose curves, profiles, penumbra width, a ngular spread in air, and effective and virtual source positions were compared. Measurements were made when the MLC alone provided collimati on and when an applicator provided collimation. Identical penumbra wid ths were obtained at a source-to-surface distance of 85 cm for the MLC and 110 cm for the applicator. The MLC-shaped beams had characteristi cs similar to other machines which use trimmers or applicators to coll imate scanned or scattered electron beams. Values of the effective sou rce position and the angular spread parameter for the MLC beams were s imilar to those of the dual-foil scattered beams of the Varian Clinac 2100CD and the scanned beams of the Sagittaire linear accelerators. A model, based on Fermi-Eyges multiple scattering theory, was adapted an d applied successfully to predict penumbra width as a function of coll imator-surface distance. (C) 1997 American Association of Physicists i n Medicine.