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An isodose shift technique for obliquely incident electron beams
Author(s) -
Ulin Kenneth,
Sternick Edward S.
Publication year - 1989
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.596316
Subject(s) - imaging phantom , optics , linear particle accelerator , beam (structure) , physics , line (geometry) , mockup , cathode ray , angle of incidence (optics) , electron , materials science , mathematics , geometry , nuclear physics , composite material
It is well known that when an electron beam is incident obliquely on the surface of a phantom, the depth dose curve measured normal to the surface is shifted toward the surface. Based on geometrical arguments alone, the depth of the n th isodose line for an electron beam incident at an angle θ should be equal to the product of cos θ and the depth of the n th isodose line at normal incidence. This method, however, ignores the effects of scatter and can lead to significant errors in isodose placement for beams at large angles of incidence. A semi‐empirical functional relationship and a table of isodose shift factors have been developed with which one may easily calculate the depth of any isodose line for beams at incident angles of 0° to 60°. The isodose shift factors are tabulated in terms of beam energy (6–22 MeV) and isodose line (10%–90%) and are shown to be relatively independent of beam size and incident angle for angles <60°. Extensive measurements have been made on a Varian Clinac 2500 linear accelerator with a parallel‐plate chamber and polystyrene phantom. The dependence of the chamber response on beam angulation has been checked, and the scaling factor of the polystyrene phantom has been determined to be equal to 1.00.