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Fermi-Eyges for electrons
After some algebra you get
Recalling that erf(x) is very close to unity for x>2, it can be seen that
N(x,y,z )=1 if both x and y are more than about 3 times the
MCS
from the
field edges, i.e. the dose profile is flat away from the edges of a broad beam,
exactly as one would expect.
The above result also implies that N (x,y,z ) is constant with depth, i.e. the
build-up due to scattering is not predicted.
The Fermi–Eyges theory do not account for electron loss.
o
An empirical correction factor is necessary in order to reproduce
measured depth–dose curves.
Warsaw 2017
10
From Handbook of Radiotherapy Physics
Ed Mayles, Nahum and Rosenwald