  We propose and analyze in detail a method to measure the in-air spatial
spread parameter of clinical electron beams. Measurements are performed at the
center of the beam and below the adjustable collimators sited in asymmetrical
configuration in order to avoid the distortions due to the presence of the
applicator. The main advantage of our procedure lies in the fact that the dose
profiles are fitted by means of a function which includes, additionally to the
Gaussian step usually considered, a background which takes care of the dose
produced by different mechanisms that the Gaussian model does not account for.
As a result, the spatial spread is obtained directly from the fitting procedure
and the accuracy permits a good determination of the angular 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 the quadratic-cubic dependence with the distance to the source predicted
by the Fermi-Eyges theory. However, the corresponding values obtained for the
scattering power are differing from those quoted by ICRU nr. 35 by a factor ~2
or larger, what requires of a more detailed investigation.
