Biomedical Engineering Reference
In-Depth Information
You will recall that a central tenet of serial architecture is that the
complication rate is a linear function of the volume irradiated.
However, in this experiment, the results strongly call that tenet into
question. It is clear from inspection of Table 5.1 that the ED
50
for
irradiation of two 4 mm sections is much closer to that for irradiation
of a single 4 mm section than it is to that for irradiation of a single
8 mm section. Of course, the serial architecture model would predict
the opposite, namely that the ED
50
for the irradiation of two 4 mm
sections would be the same as that for irradiation of the 8 mm section
since, in that case, the volume irradiated would be the same.
They undertook an even more provocative experiment, which
has become known as a “bath-and-shower” experiment. In this
experiment (Bijl
et al
.,
2003) a 20 mm length
of the rat spinal cord
(the so-called “bath”)
was irradiated to a
sub-threshold dose,
7
and a short 4 mm
shower
shower
shower
bath
bath
bath
brain
brain
brain
Bath dose
(Gy)
Bath dose
(Gy)
Bath dose
(Gy)
Bath dose
(Gy)
ED
50
(Gy)
ED
50
(Gy)
ED
50
(Gy)
ED
50
(Gy)
95%
CL
95%
CL
95%
CL
95%
CL
0
0
0
0
54
54
54
54
49-62
49-62
49-62
49-62
4
4
4
4
39
39
39
39
37-40
37-40
37-40
37-40
12
12
12
12
33
33
33
33
32-35
32-35
32-35
32-35
length of the cord in
18
18
18
18
31
31
31
31
26-35
26-35
26-35
26-35
the center of the bath
field (the so-called
-10
-10
-10
-5
-5
-5
0
0
0
5
5
5
10
10
10
1
1
1
distance (mm)
distance (mm)
distance (mm)
“shower”) was ir-
radiated to graded
Figure 5.10. The “bath-and-shower” experi-
ment on rat spinal cord (Bijl
et al
., 2003),
for paralysis in the shower
doses and the ED
showing the ED
region as a function of the bath dose (see text).
50
50
of the central 4 mm
section of cord (bath
plus shower dose) was
determined. This experiment was repeated for several different bath
doses, as is shown in Figure 5.10.
When no bath is applied, the ED
50
for paralysis of the fore or hind
limbs is 54 Gy. When a sub-threshold 18 Gy bath is added, the ED
50
is sharply reduced to 31 Gy. Even when the bath dose is as low as
4 Gy, the ED
50
reduction!
is still greatly reduced - from 54 to 39 Gy, a 28%
7
The ED
50
for irradiation of a 20 cm length of cord is 20.4 Gy, and the slope
of the dose-response curve is very steep, such that doses below about
18 Gy to a 20mm length of cord would not lead to paralysis.
