Influence of cone design on the electron beam characteristics on clinical accelerators

Electron beam characteristics were investigated for different applicator cone designs. Low-energy scattered electrons from the cone housing can contribute to the skin dose. The depth dose was measured as a function of electron energies and field size for different cones. The measurements were compared with calculated 3-D dose distributions obtained from Monte-Carlo-derived energy deposition kernels [Phys. Med. Biol. 37, 391 (1992)]. This comparison shows that beam-defining cones can be fabricated to eliminate low-energy scattered electrons from the cone housing. Eliminating low-energy electrons from the cones provides depth dose (DD) characteristics for clinical electron beams that are comparable to those (1) from nearly monoenergetic electrons (as determined from the practical range of the electrons), and (2) for which the DD is nearly independent of source-to-skin distance. Measurements of DD for small circular fields were also performed. For very small field sizes, corresponding to the width of the lateral spread of a pencil beam, the theoretical and measured DD were found to depend on field size. Three-dimensional dose calculations of small and irregularly shaped electron fields are necessary to appreciate these effects.