Biomedical Engineering Reference
In-Depth Information
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R
ADIATION
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Radiation can cause lethal damage to cells, mainly by forming highly
reactive radicals in the intracellular material that can chemically break
bonds in DNA, causing a cell to lose its ability to reproduce. The
higher the dose, the greater the probability of sterilizing cells. Such
damage is experienced both by the malignant cells one is trying to
eradicate, and by the cells in the healthy tissues that receive radiation
even though one would wish to spare them. There are two elements
to the strategy for maintaining the functional competence of the
irradiated normal tissues and organs.
First, there appears to be a small and favorable difference between
the radiation response of normal and malignant cells that allows
preservation of the normal cells that permeate the tumor, and of the
1
nearby tissues that are included in the target volume. The reasons
for this difference are complex, not fully understood, and even
controversial. The difference is probably due less to differences in
intrinsic cellular radiosensitivity than to differences in the genetic
machinery activated by radiation, in DNA repair kinetics, and in the
mechanisms of cell repopulation - and is counterbalanced by tumor-
protective factors such as the substantially greater resistance to
radiation of cells in regions of low oxygen tension such as are often
found in tumors. To further the differential effect, the dose is usually
delivered in small daily increments, termed fractions. This strategy is
generally thought to improve substantially the therapeutic advantage
as compared with radiation delivered in a single application. Con-
sequently, in conventional radiotherapy, about 30 to 40 daily
fractions of approximately 2 Gy each are used.
2
These fractions are
typically delivered once a day, with a weekend break, so that a course
of radiotherapy will typically last from 5 to 8 weeks. Treatment may
also be accelerated, for example, by delivering two fractions per day,
or by delivering higher doses per fraction with fewer fractions.
1
One generally defines as the target a volume that includes demonstrable
disease, possible subclinical extension of that disease (delineating this is
one of the radiation oncologist's arts), and a safety margin for organ and
patient motion and technical uncertainties. This is termed the planning
target volume (PTV) as more fully described in Chapter 3.
2
There are particular clinical situations, usually involving relatively small
target volumes, in which far fewer fractions, sometimes only one, are
employed.
