CHAPTER 8
274
A typical high energy linac may produce electron beams with
discrete energies in the range from 4 to 25 MeV.
Electron beams can be considered almost monoenergetic as they
leave the accelerator; however, as the electron beam passes through
the accelerator exit window, scattering foils, monitor chambers,
collimators and air, the electrons interact with these structures,
resulting in:
A broadening of the beams electron energy spectrum;
Bremsstrahlung production contributing to the bremsstrahlung tail
in the
electron beam percentage depth dose (PDD) distribution.
On initial contact with the patient, the clinical electron beam
has an incident mean energy E
0 that is lower than the electron energy inside the
accelerator.The ratio of the dose at a given point on the
central axis of an electron
beam to the maximum dose on the central axis multiplied by 100
is the PDD, which is normally measured for the nominal treatment
distance (i.e. the distance between the accelerator exit window and
the patients skin) and depends on field size and electron beam
energy.
8.1.2. Electron interactions with an absorbing medium
As electrons travel through a medium, they interact with atoms
by a variety of Coulomb force interactions that may be classified
as follows:
0 5 10
110 100
90 80 70 60 50 40 30 20 10
0
6 18 MeV 12 9
Depth in water (cm) Depth in water (cm)
(a) (b)110 100
90 80 70 60 50 40 30 20 10
00 5 10 15 20 25
15 MV
6 MV
PP
D (%
)
PP
D (%
)
FIG. 8.1. Typical central axis PDD curves in water for a 10 10
cm2 field size and an SSDof 100 cm for (a) electron beams with
energies of 6, 9, 12 and 18 MeV and (b) photonbeams with energies
of 6 and 15 MV.
