Biomedical Engineering Reference
In-Depth Information
undesirable patient reactions until the problem was appreciated and
cured. This was one of the not-infrequent situations in which dosi-
metric problems were first discovered by clinicians, due to un-
toward patient reactions and underlines out how sensitive the patient
is to the dose he or she receives.
T
HE
D
ESIGN OF A
U
NIFORM
R
ECTANGULAR
T
REATMENT
B
EAM
Let us jump ahead a little, and describe the formation of a simple
beam of photons. Figure 4.11
depicts this schematically. In
this figure, the beam delivery
system (which is usually
mounted on a rotating gantry) is
comprised of everything within
the trapezoid region, and the
medium being irradiated is, in
essence, a bucket of water.
For the moment, let us assume
that the collimator is a square
hole in a metal block, and that
there is no patient-specific
aperture or intensity-modifying
device in the beam. I want to
discuss now what the dose
distribution looks like along the
two dotted blue lines shown in
the figure.
Figure 4.11. Schematic representa-
tion of a radiation beam impinging
on a bucket of water.
Distribution of the dose in depth
Figure 4.12 shows a sequence of semi-logarithmic depth dose curves,
taken along the central axis of the beam, in which various physical
effects are “turned on” in successive panels of the figure. We now
discuss these effects.
When photons impinge on matter, their number gets attenuated as
depth increases, simply due to the loss of photons from upstream
Compton interactions. And, as their number decreases, the dose that
they deposit decreases proportionately. Less photons create less
“splashes”. This attenuation is, to a first approximation, exponential
with depth. That is, one can write:
