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About
σ
❑
The lateral spread (or
σ
) increases with depth until a maximum spread is
achieved.
❑
Beyond this depth there is a rapid loss of electrons as their larger lateral
excursion causes them to run out of energy.
❑
σ
increases with depth indefinitely using Fermi-Eyges theory, which is
contrary to what is observed experimentally in a narrow-beam dose
distribution.
❑
σ
is also over-estimated since electrons who have been thru large angle
scatter events at shallower depths and lost due to loss of energy are ignored
(Barry Med Phys 1982).
Dublin 2018
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