Biomedical Engineering Reference
In-Depth Information
One can understand this striking behavior in terms of the
representation of serial architecture as a chain, as sketched in Figure
5.7. Imagine that the irradiation field just covers one link of the
chain, and let us assume that the dose is such that there is a 10%
chance that the link will break - thus causing the chain to fail under
load. In that case, the NTCP will be 10%. Now, let us double the
field size so that two links are in the field, both receiving the same
dose as before. Each link will have a 10% chance of being broken, so
the chance that the chain will break under load (the NTCP) is roughly
the sum of these probabilities, namely 10% + 10% = 20%.
6
This
linear relationship is absolutely basic to serial models. It is
sometimes said that “serial architecture tissues show no volume
effect,” by which is meant that the dose to achieve a given NTCP is
essentially independent of volume. However, the argument just given
shows that such a statement cannot be precisely true.
Parallel architecture
Normal tissues exhibiting
a parallel architecture are
also assumed to consist
of FSUs, each of which
performs the function
that the normal tissue
is responsible for. How-
ever, rather than losing
function when any one
FSU is lost, a parallel
Figure 5.8. Parallel architecture: normal
tissue viewed as a rope which loses its
functionality (i.e., load-bearing capacity)
when a critical number of its strands are
broken.
structure is thought to
be able to maintain its
function provided some
critical fraction of the
FSUs (e.g., 30%) maintain their function. Only when the damage to
the FSUs is so great that the necessary critical fraction of them is not
preserved, does the tissue itself lose functionality. The kidney and
lung, for example, are thought to be parallel structures with the FSUs
being, respectively, nephrons and alveoli. A parallel structure can be
thought of as something like a rope, comprised of many strands, as
sketched in Figure 5.8. The rope can support a load as long as a
6
More accurately, following equation (5.3), we have (1-NTCP) =
(1-10%)
⋅
(1-10%) = 81% . That is, the NTCP will actually be 19%.
