Biomedical Engineering Reference
In-Depth Information
ment of the tumor cells in prescribing the total dose, the number of fractions, the
dose per fraction, and the total treatment time.
Because cells exhibit different degrees of radiosensitivity in different phases of
the cell cycle, an asynchronous cell population will become partially synchronized
by irradiation. The surviving cells will generally be those in the more resistant
phases. As the population continues to grow following exposure, the partially syn-
chronized surviving cells become redistributed over the complete cycle, includ-
ing the more sensitive phases. This process of redistribution, combined with re-
peated irradiation at intervals, tends to result in increased cell killing relative to
that achieved with a single dose.
The oxygen effect is extremely important in radiotherapy. Tumors often have
poorly developed blood vessels, intermittent blood flow, and clonogenic cells with
greatly reduced oxygen tension. They contain regions with viable cells, which are,
however, hypoxic and therefore relatively resistant to radiation (cf., Fig. 13.18). De-
livering radiation to a tumor in fractions allows reoxygenation of some hypoxic
cells to occur between doses. As an added factor, sensitization increases rapidly
with oxygen tension. The result is greater killing of tumor cells than with a com-
parable single dose. The response of the normal, oxygenated cells is unchanged by
this procedure.
13.15
Suggested Reading
A vast amount of information is readily available on the World Wide Web, as well
as complete copies (some searchable) of several of the works listed below.
1
Baverstock, K. F., and Stather, J. W.,
eds.,
Low Dose Radiation, Biological
Bases of Risk Assessment
,Taylorand
Francis, London (1989). [This valuable
book of 606 pages contains 54 contri-
butions, covering all essential aspects
of its subject. The articles are from
scientists in a number of disciplines.]
2
BEIR V,
Health Effects of Exposure
to Low Levels of Ionizing Radiation:
BEIR V
, Committee on the Biological
Effects of Ionizing Radiation, National
Research Council, National Academy
Press, Washington, DC (1990).
3
BEIR VI,
The Health Effects of Exposure
to Indoor Radon: BEIR VI
, Committee
on the Biological Effects of Ionizing
Radiation, National Research Council,
National Academy Press, Washington,
DC (1999).
4
BEIR VII,
Health Risks from Exposure
to Low Levels of Ionizing Radiation:
BEIR VII-Phase 2
, Committee on
the Biological Effects of Ionizing Ra-
diation, National Research Council,
National Academy Press, Washing-
ton, DC (2006). [Reviews health risks
from exposure to low levels of low-
LET ionizing radiation. Updates BEIR
V. Detailed risk estimates for both
cancer incidence and cancer mortality
are given. Required reading.]
5
Glass,W.A.,andVarma,M.N.,eds.,
Physical and Chemical Mechanisms in
Molecular Radiation Biology
,Plenum
Press, New York, NY (1991). [A collec-
tion of 18 papers at an invited work-
shop on the subject. Contributions
are made from fields of radiologi-
cal physics, radiation chemistry, and
radiation-effects modeling.]
6
Gusev, Igor A., Guskova, Angelina,
and Mettler, Fred A., eds.,
Medical
Management of Radiation Accidents
,
