Scattering foil device for high-energy electron-beam therapy.

Abstract

During the past decade, treatment-planning for high-energy electron-beam therapy has progressed along various lines to achieve homogeneous irradiation of tumor, with minimum damage to surrounding normal tissue. One of the most frequently used methods is an absorption technic in which a water-equivalent substance is interposed in the end of a collimator system (8, 11, 13, 14, 16–18). The distribution of beam intensity is thus well adjusted to the individual case for any single fixed field. With the absorbing material in the end of a treatment cone, however, the limited skin-sparing effect is lost because of secondary radiation. The benefits of high-energy radiation without the disadvantage of such absorbing material are desirable. This is a report of an evaluation of a special device consisting of scattering foil to alter the intensity of the betatron electron beam. Isodose curves for clinical use were measured and compiled. Method A betatron3 operating in the 4 to 31 MeV energy range was used in this study. Its electron beam emerged with a stability of ±7 per cent according to a Radocon ionization chamber. Initially, the direction of the beam emerging from the “doughnut” was precisely adjusted to the center of the treatment field by a deflection magnet between the “doughnut” window and the scattering foil. The beam passing through the scattering foil was collimated by a lead shield and aluminum-walled treatment cone. The isodose distributions in a water phantom were measured with an ionization chamber, which has a 4-mm diameter air volume with a 5-mm-thick Lucite wall. The ionization chamber was connected to an automatic isodose plotter. Scattering foils were fixed transversely in an 11-step ladder applicator in front of the deflection magnet. The individual foils could be changed outside the betatron head. In addition to those regularly used, more than 5 different foils could be affixed in this ladder system, and any of them readily changed as clinically indicated (Figs. 1 and 2). Foils with several configurations, including concave, wave, and sieve were evaluated. Only bilateral metal foils with several millimeters separation in this ladder system produced ideal isodose curves. Nickel and lead foils, each 0.2 mm thick, and a lead foil 0.4 mm thick were evaluated at 10 MeV, 16 MeV, and 25 MeV. Their composition is shown in Figure 3. The position and separation of foils were adjusted for proper isodose distribution in the 90 and 70 per cent regions. Results The isodose curves shown in Figures 4–7 have but a limited width in the 90 per cent region surrounded by a broad extent of the 70 per cent region. The position and width of the 90 per cent curve in a plane horizontal to body surface could be altered as required by adjusting the space between the foils without change in the flatness of the 70 per cent curve. This desirable effect was not readily achieved in lower energy ranges, around 10 MeV.