Biomedical Engineering Reference
In-Depth Information
Target
Beryllium
window
Ionisation
chamber
Mirror
Mirror
Fig. 19.1
Drawings of: Varian Clinac 2100 C/D operating in photon mode at 18 MV (upper left)
and electron mode at 6 MeV (upper right), Elekta MLCi operating in photon mode at 10 MV (lower
left) and electron mode at 4 MeV (lower right)
start from that position in the linac head, assuming as primary electron source a
pencil beam with given spatial and energy distributions. Particles are then simulated
downstream of the linac head. Therefore, from a Monte Carlo simulation point of
view, the relevant constructive elements of the linac are those found downstream of
the primary electron source.
Some linacs operate only with electron beams (e.g., Siemens Mevatron ME),
others with photon beams (e.g., Varian Clinac 600 C/D), while others can operate
either with electron or photon beams (e.g., Varian Clinac 2100 C/D). Those irradiat-
ing with electron beams usually include some thin material layers downstream of the
primary source, called scattering foils, whose purpose is to spread the pencil beam
and hence to cover a large field. Linacs irradiating with photon beams have a thick
material target, usually made of tungsten, just downstream of the primary electron
source. This target produces photons by bremsstrahlung emission. In many cases a
flattening filter is placed in the position of the scattering foils in order to homogenize
the energy distribution of the emitted photons. Downstream of the aforementioned
constructive elements a series of collimating structures are found whose purpose is
to conform the beam to the required field shape. Figure
19.1
shows four drawings of
the constructive elements of the Varian Clinac 2100 C/D and Elekta MLCi operating
in photon and electron modes.
