A method to determine the in-air spatial spread of clinical electron beams

We propose and analyze in detail a method to measure the in-air spatial spr ead parameter of clinical electron beams. Measurements are performed at the center of the beam and below the adjustable collimators sited in asymmetri cal configuration in order to avoid distortions due to the presence of the applicator. The main advantage of our procedure lies in the fact that the d ose profiles are fitted by means of a function which includes, in addition to the Gaussian step usually considered, a background which takes care of t he dose produced by different mechanisms for which the Gaussian model does not account. As a result, the spatial spread is obtained directly from the fitting procedure and the accuracy permits a good determination of the angu lar spread. The way the analysis is done is alternative to that followed by the usual methods based on the evaluation of the penumbra width. Besides, the spatial spread found shows quadratic-cubic dependence with thr distance to the source predicted by the Fermi-Eyges theory. However, the correspond ing values obtained for the scattering power are differing from those quote d by ICRU nr. 35 by a factor of similar to 2 or larger, which requires of a more detailedinvestigation. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)01404-2].