Biomedical Engineering Reference
In-Depth Information
5.
B
IOLOGY
M
ATTERS
Introduction ..........................................................................................85
Models ..................................................................................................87
Established experience ............................................................................87
Therapeutic ratio .....................................................................................88
Types of models ......................................................................................88
Skepticism concerning models ...............................................................90
Mechanistic vs. empirical models...........................................................91
Dose-Volume Models for Tumors ........................................................91
TCP and minimum dose..........................................................................92
TCP: mechanistic models........................................................................93
EUD: an empirical model .......................................................................96
Dose-Volume Models for Normal Tissues ...........................................98
NTCP: mechanistic models.....................................................................99
EUD: an empirical model .....................................................................103
Caveats ...............................................................................................104
Caveats concerning models of dose-volume effects of tumors ...........104
105
Caveats concerning models of dose-volume effects of normal tissues ..
Summary .............................................................................................110
I
NTRODUCTION
∗
When tissues are irradiated, a complex and not fully understood
chain of events takes place. At the highest level one can simply say
that the radiation interacts with the tissues “through physics”. Then
chemistry takes over, followed by biology. This progression is
illustrated in Figure 5.1.
Dose has always been the meeting ground between radiation
oncologists and physicists and, indeed, between radiation oncologists
with one another. The one asks for a given dose or dose distribution
to be delivered to the patient; the other provides it. When asked to
explain my work, I often say that we physicists are the pharmacists of
∗
It takes considerable courage even to address the subject of the biology of
radiation therapy, given the availability of Eric Hall's fine textbook
(Hall, 2000) - I am very happy to acknowledge its, and his, influence on me.
85
