Biomedical Engineering Reference
In-Depth Information
These results strongly suggest that the radiation experience of tissues
adjacent to a high dose region can markedly affect the radiation
tolerance of that region. This dependency is not consistent with the
serial model as formulated to date.
“Parallel” architecture
Tissues having a parallel architecture are assumed to be constituted
of a large number of FSUs, all having the same radiosensitivity.
However, in the lung for example, which is thought to be a good
example of parallel architecture, Travis and colleagues have shown
that it can be otherwise (Liao
et al.
, 1995). They developed a
technique that allowed them to selectively irradiate mice, using
careful collimation, delivering dose to only a third of the lung in the
cephalad-caudad direction. They irradiated the top, middle and
bottom thirds of the lung and assayed for both breathing rate and
death from radiation pneumonitis. The radiation sensitivity of the
bottom third of the lung was significantly greater than that of the apex
of the lung in both assays.
There are several experiments that indicate that
the environment in the
neighborhood of a region of high dose can markedly affect the
response of tissues to radiation
. In what follows, I briefly summarize
a few of these observations.
Lung damage as a function of heart irradiation
In models of
tissues with a parallel architecture, the tissue compartment or organ is
assumed to react to its irradiation independently of what is going on
around it. An interesting experiment (van Luijk
et al
., 2005) calls this
very much into question,
at least in the system
these authors studied.
Using careful collimation,
they were able to irradiate
specific regions in rats,
namely: the heart, the
heart plus medial lung,
lateral lung on both sides,
and heart plus lateral lung
on both sides.
Using breathing rate as
their endpoint, they were
able to compare,
inter
alia
, the response to
80
lung+heart
lung only
60
40
20
0
16
18
20
22
dose (Gy)
Figure 5.11. Rat breathing rate when the
lateral lungs are irradiated with (red) and
without (blue) the heart being irradiated.
Data from van Luijk
et al.
(2005)
