Biomedical Engineering Reference
In-Depth Information
radiotherapy. The medication we dispense is radiation; we are
responsible for ensuring that the patient is given the prescribed dose
of the prescribed quality. But, and I cannot emphasize it too much,
dose is only a surrogate for what is clinically important
- to the
patient and to his or her therapists. What we care about is cure and
morbidity; to achieve the one and to avoid the other, as far as
possible. Our goal is biological.
PHYSICS
PHYSICS
CHEMISTRY
CHEMISTRY
BIOLOGY
BIOLOGY
ionization of
atoms;
possible direct
breakage of one
or both DNA
strands
ionization of
atoms;
possible direct
breakage of one
or both DNA
strands
creation of free
radicals and
other chemical
changes
creation of free
radicals and
other chemical
changes
damage to DNA
and other intra-
cellular and,
possibly, extra-
cellular targets
damage to DNA
and other intra-
cellular and,
possibly, extra-
cellular targets
cell death
(microscopic)
cell death
(microscopic)
generation of heat
(about 96% of dose)
generation of heat
(about 96% of dose)
tumor control and
organ damage
(macroscopic)
tumor control and
organ damage
(macroscopic)
Figure 5.1. Block diagram suggesting the sequence
of events following the irradiation of tissues, passing
from physical effects, through chemical effects, to
cellular and eventually tumor and normal tissue
damage. The start and endpoints are highlighted.
In fact, it is amazing how good a surrogate dose is - especially given
that so little of it goes into chemical change, as mentioned in
Chapter 4. But, having said that, there are many important factors
that modulate the clinical effect resulting from a given dose. To name
but a few:
the fractionation scheme (e.g., the inter-fraction interval, dose
per fraction, etc.);
the inherent radiosensitivity of the tumor and normal tissues;
the radiation sensitizing effects of concomitant disease (e.g.,
diabetes), concomitant chemotherapy, genetic differences (e.g.,
ataxia telangiectasia), and so forth;
the degree of oxygenation of the cells of the tumor and normal
tissues;
the way dose is distributed within the tumor and normal tissues.
