SHARPENING THE PENUMBRA OF HIGH-ENERGY ELECTRON-BEAMS WITH LOW-WEIGHTNARROW PHOTON BEAMS

Background and purpose: High energy (20-50 MeV) electron beams, availa ble from the MM50 Racetrack Microtron, can be used for the treatment o f deep-seated tumors. A disadvantage is the increasing penumbra width as a function of depth. By the addition of a narrow (typically 1 cm wi de) photon beam near the field edge, the 50-90% penumbra width of the electron beam is reduced, yielding a significantly increased effective field size. Materials and methods: For rectangular electron beams in a water phantom (energies 25 and 40 MeV, field sizes 5 x 5-15 x 15 cm( 2)) a computer program was used to optimize the photon beam parameters (position, weight and width) to obtain a combined beam with the sharp est penumbra at the optimization depth and a beam flatness within cert ain constraints. The study furthermore included penumbra sharpening of an irregular multileaf collimator-shaped field. Results and conclusio n: At optimization depths near Rw, photon beam addition reduces the pe numbra width by 40-50% (from 15-20 mm to 8-10 mm). Beam flatness at th e optimization depth is within +/-5% and hot-spots are less than or eq ual to 120% for all depths. By the addition of narrow photon beams aro und the rectangular or irregular field, the electron field width can b e reduced by 1-3 cm, while the effective field size is maintained. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.