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The widening of the pencil is limited
The pencil-beam width (
MCS
)
as a function of depth agrees
well with experiment at small
and moderate depths.
Then it continues to increase,
whereas in reality it goes
through a maximum and
finally decreases.
This is managed by the
g(x)
function or has to managed in
other ways in some systems
This is due to the reduction of
the number of electrons in the
beam at large depths due to
range straggling
Warsaw 2017
13
From Handbook of Radiotherapy Physics
Ed Mayles, Nahum and Rosenwald