Biomedical Engineering Reference
In-Depth Information
How this
dose build-up
comes about is schematically illustrated in
Figure 4.13. In this figure, a set of photons (dot-dash arrows) impinge
on a block of material and they have their first interactions at
successively greater depths. Each then lets loose an electron (red
arrow), which travels in the forward direction for a fixed distance and
then stops. The number of electrons at a given depth in this example
is shown for several depth
levels. This number begins
from zero, then increases
incrementally until reaching an
“equilibrium” value of six. In
practice, of course, there is a
huge number of photons, the
electrons they produce have a
range of energies, and the
electrons do not all travel in
the forward direction. While
this complicates the picture
slightly, the same principle
build-up of dose until an
equilibrium level is reached at a depth strongly related to the average
distance traveled by secondary electrons (e.g., a few millimeters up to
a few centimeters for very high energy photons). The dose reaches a
maximum value, due to the counter-acting effects of dose build-up
and exponential photon attenuation.
In this idealized example, the dose at the very surface of the material
is essentially zero. The estimate of zero entrance dose is due to the
assumption that all secondary electrons travel forward. In fact, the
angular distribution of electrons from Coulomb interactions has a
wide spread of angles, including a minority of electrons which, due to
scattering effects, travel in the backward direction. These, together
with contaminants in the beam such as electrons emanating from the
collimator edges or material in the beam, raise the entrance dose to
some tens of percent of the maximum dose. How much they raise it
is a function of photon energy and field size.
As a result of the build-up phenomenon, high energy photons have a
low entrance dose. In patients, this so-called
skin-sparing effect
is
Figure 4.13. Schematic explanation
of build-up effect (see text).
applies: there is a gradual
extremely important in avoiding high doses to the epidermis. In
earlier times, when only low energy photon beams were available
