Electron beam and virtual SSD

Electron beams are used with external collimating devices known as
cones or applicators that reduce the spread of the electron beam in the air. The
design of these cones is dependent on the manufacturer and affects the
dosimetric properties of the beam.
Electron shielding for irregular fields may be accomplished with the use
of thin lead or low melting point alloy inserts. These shielding inserts can have
significant effects upon the electron beam dosimetry (especially the PDD and
output), and these effects may be modelled by the TPS.
The design of the linac head may be important for electron dosimetry,
especially for Monte Carlo type calculations. In these conditions particular
attention is paid to the scattering foil. The effective or virtual SSD will appear
to be shorter than the nominal SSD, and may be taken into consideration by
the TPS.

Podgorsak