I looked it up on my blog and had posted from an unknown reference a few years ago and never included the information.
8.1.3. Inverse square law (virtual source position)
In contrast to a photon beam, which has a distinct focus located at the
accelerator X ray target, an electron beam appears to originate from a point in
space that does not coincide with the scattering foil or the accelerator exit
window. The term ‘virtual source position’ was introduced to indicate the
virtual location of the electron source.
The effective source to surface distance (SSD) for electron beams
(SSDeff) is defined as the distance from the virtual source position to the point
of the nominal SSD (usually the isocentre of the linac). The inverse square law
may be used for small SSD differences from the nominal SSD to make
corrections to the absorbed dose for variations in air gaps between the patient
surface and the applicator.
There are various methods to determine the SSDeff. One commonly used
method consists of measuring the dose at various distances from the electron
applicator by varying the gap between the phantom surface and the applicator
(with gaps ranging from 0 to 15 cm). In this method, doses are measured in a
phantom at the depth of maximum dose zmax, with the phantom first in contact
with the applicator (zero gap) and then at various distances g from the
applicator. Suppose I0 is the dose with zero gap (g = 0) and Ig is the dose with
gap distance g. It follows then from the inverse square law that:
