Biomedical Engineering Reference
In-Depth Information
probability can make use of doses adjusted for fractionation effects -
the so-called biologically effective dose (York, 2003) - but this
adjustment certainly does not tell the whole picture.
S
UMMARY
In this chapter I have tried to walk a tight line between, on the one
hand, emphasizing the desirability of having explicit models of dose-
volume effects in tissues and, on the other hand, citing a number of
examples where the current models are inadequate. The bottom line
is that such models can be useful as an aid to planning and analyzing
treatments, but should never be relied upon uncritically. In particular,
one should be extremely cautious when they point to approaches
outside of established experience.
The examples have in part been chosen to emphasize that the
environment around a high-dose region of a normal tissue can be of
great importance in predicting its radiation response. Low to medium
doses may appear both within an organ and in its vicinity. I am
particularly concerned that we not focus only on the high-dose
volume, discounting the adjacent regions of lower dose.
I entitled this chapter “Biology Matters” for a very simple reason.
Namely, that the biological consequences of irradiating both the
tumor and normal tissues really do matter very much so far as the
success of radiation therapy, or lack thereof, is concerned. Although
this is obvious, I have hardly ever heard radiobiological con-
siderations being explicitly invoked when physicists and physi-
cians sit down together to develop a plan of treatment for a specific
patient. This may, in large part, be due to insecurity about how
to convert biological issues into a prescription - and whether there
are sufficient data to do so. If so, I am sympathetic, but not in
agreement. The fact that something important is hard does not relieve
one from the obligation to do the best one can about it. With or
without explicit models, we make radiobiological judgments many
times each day in radiation therapy. It behooves us to make them
explicit. This will allow comparison of data and experience, and is
the path to further understanding. The growth of activity in the last
two decades relating to the measurement and prediction of TCP and
NTCP attests to this.
