A small increase in nominal SSD will significantly affect what aspect of an electron beam?

A small increase in nominal source-to-surface distance (SSD) significantly affects the central axis depth dose for an electron beam because this distance directly influences how the beam interacts with tissue as it penetrates.

As the SSD increases, the energy of the electrons is deposited at different depths within the tissue. This is due to the fact that electrons have a limited penetration capability and their dose distribution is heavily dependent on the geometry of the beam and the distance it travels through tissue. With an increase in SSD, the dose delivered to the surface may decrease because the electrons scatter and lose energy as they travel further from the source, thereby affecting the central axis depth dose.

This aspect is crucial for treatment planning, as accurate dose calculations ensure that the prescribed dose reaches the intended target volume while minimizing exposure to surrounding healthy tissues. Understanding this relationship helps dosimetrists adjust treatment parameters accordingly to achieve optimal therapeutic outcomes.