Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5196_Библиотеки_им_академика_М_И_Перельмана.pdf
CHAPTER 8 Early Tissue Reactions and Their Effects on Organ Systems
147
Effects on the Reproductive System
Some of the early tissue reactions from ionizing radiation
have been discussed in Chapter 7.
Human germ cells are relatively radiosensitive. Doses
as low as 0.1 Gyt can depress the male sperm population,
and this same dose has the potential to cause genetic
mutations in future generations. In females, a gonadal
dose of 0.1 Gyt may delay or suppress menstruation.
Animal experiments and data from irradiated human populations have provided important information
on gonadal response to radiation exposure. Irradiated
human populations include:
• Patients who have undergone radiation therapy
• Radiation accident victims
• Volunteer convicts
The testes of the male and the ovaries of the female
do not respond in the same manner to irradiation because of the differences in the method in which these
cells are produced and progress from elementary stem
cells to mature cells. The spermatogonia, the stem cells
of the testes, continually reproduce. They evolve and
become spermatocytes. The latter cells then multiply
and develop into spermatids that eventually differentiate and become spermatozoa, or sperm, which are the
functionally mature germ cells (Fig. 8.8). The development of the male stem cell into a functionally mature
germ cell takes 3 to 5 weeks.
In the female, the oogonia, the ovarian stem cells,
multiply to millions of cells only during fetal development, before birth, and then they steadily decline in
number throughout life. During the latter part of fetal
development, the oogonia become encapsulated by numerous primordial (primary) follicles that grow around
them (see Fig. 8.8). The oogonia then become oocytes,
Male:
Spermatogonia
Female:
8,9
5
Spermatocyte Spermatid Sperm
which contain follicles that are nests of cells, some of
which eventually mature during the reproductive life of
a woman. Before these primary oocyte-containing follicles grow into mature follicles, they remain dormant
until puberty. Then, just before puberty, the oocytes are
reduced in number to only several hundred thousand.
Some of the cells of the primary follicles proliferate in
response to stimulation by hormones from the pituitary
gland, and these cells begin to mature. At the same time,
the ovum contained within each of the follicles undergoes meiosis. As puberty begins, the developed ova,
or mature female germ cells*, within the follicles are
ejected when the follicles themselves rupture. Usually,
only one follicle will fully mature and move toward the
surface of the ovary to be expelled, and the others disintegrate. This process occurs at regular time intervals of
approximately 28 days. Of the mature ova that are actually enclosed within the follicles, only 400 to 500 are
produced, matured, and made available for fertilization
during a woman’s reproductive life.
Follicles range in size from small to large. Of these,
the intermediate-size follicles are the most radiosensitive, and the small follicles are the least radiosensitive.
Large, mature follicles possess only a moderate degree
of radiosensitivity.10 During the female menstrual cycle,
a mature follicle releases an ovum during the period of
ovulation, when a ripe egg is expelled from an ovary
into the pelvic cavity. If a waiting male sperm does not
fertilize that ovum in the uterus, it will be lost during
menstruation and not replaced.
Hematologic Effects
As a brief review, recall that during the 1920s and 1930s,
periodic blood counts were the only means of radiation
exposure monitoring for radiation workers engaged in
radiologic practices. The use of personnel dosimeters for
monitoring occupational exposure made the simplistic
practice obsolete. When blood counts were used to
monitor the effects of radiation exposure among those
who worked with radiation, a whole-body radiation
dose of 0.25 Gyt would be required to produce a measurable hematologic depression. Such a dose could cause
enough of a decrease in the number of lymphocytes in
Primordial
follicle
Fig. 8.8 Development of the germ cell from the stem cell
phase to the mature cell.
Mature follicle Corpus luteum Ovum
*Mature female germ cells or eggs are those that have entered
stage M2 of meiosis. The end result of meiosis is the halving
of the number of chromosomes and genetic material.
148
CHAPTER 8 Early Tissue Reactions and Their Effects on Organ Systems
the blood to render the body vulnerable to infection
by foreign invaders. This method therefore, was completely unfounded for modern principles of radiation
protection.
Hematopoietic System. The hematopoietic system
consists of:
• Bone marrow
• Circulating blood
• Lymphoid organs (lymph nodes, spleen, and thymus
gland)
Cells of this system develop from a single precursor
cell, the pluripotential stem cell. The following are
other types of cells that originate from this one type of
primary cell: lymphocytes, granulocytes, thrombocytes
or platelets, and erythrocytes. Fig. 8.9 demonstrates the
progressive development of these cells from a single
pluripotential stem cell. Most of these blood cells are
manufactured in bone marrow at different intervals,
and when they mature, they enter the blood capillaries
and the peripheral circulation. Even though blood cells
are continually being produced, the life span of each
type of blood cell differs, varying, on average, from only
a few hours (e.g., lymphocytes) to almost 120 days (e.g.,
erythrocytes).
The human body may experience health-related con-
sequences throughout life if there is a decrease in the
numbers of these various cells. Some of these consequences will be increased susceptibility to aggressive
infectious organisms, higher risk of hemorrhage, and
anemia.
Radiation doses resulting from diagnostic imaging
procedures during which appropriate radiation protection methods have been employed for patients and all
personnel result in negligible damage to the blood and
the blood-forming organs. However, in this dose range,
some chromosomal changes in circulating lymphocytes
have been observed.
Cytogenetic Effects
In simple terms, cytogenetics may be defined as the
study of cell genetics with an emphasis on cell chromosomes. The techniques used to study and observe the
chromosomes of each human cell have contributed significantly to advancing genetic analysis and the understanding of the influence of radiation on genetics.
Cytogenetic analysis of chromosomes may be accomplished through the use of a chromosome map
called a karyotype. This map consists of a photograph
or photomicrograph, of the human cell nucleus during
metaphase, when each chromosome can be individually
perceived. The karyotype is constructed by extracting
the individual chromosomes and pairing them on the
map with their sister chromosomes. These chromosome
pairs are usually aligned by size, beginning with the
largest pair and ending with the smallest pair (Fig. 8.10).
Metaphase is the phase of cell division in which
chromosome damage caused by radiation exposure can
Reticulocyte Pronormoblast
Erythrocytes
Stem cell
Myeloblast
Neutrophilic
granulocytes
Fig. 8.9 Progressive development of various cells from a single pluripotential stem cell.
Eosinophilic
granulocytes
Megakaryoblast
Platelet-producing
megakaryoblast
Lymphoblast
Thrombocytes
(platelets)
Lymphocyte
CHAPTER 8 Early Tissue Reactions and Their Effects on Organ Systems
149
A
1
C
6
D
13
F
19
Fig. 8.10 A photomicrograph of the human cell nucleus at metaphase that shows each chromosome indi-
vidually demonstrated. The karyotype is constructed by cutting out the individual chromosomes and pairing
them with their sister chromosomes. These chromosome pairs are usually aligned by size, beginning with
the largest pair and ending with the smallest pair. The left karyotype is male, and the right is female. (From
Carolyn Caskey Goodner, Identigene, Inc.)
2
8
7
14
20 21 22
15
G
B
3
9 10
E
16
4
11
17 18
Sex
chromosomes
X Y
5
12
be evaluated. Chromosome aberrations (deviation from
normal development or growth of structures that contain genetic material) and chromatid aberrations have
been observed at metaphase.
Both low and high radiation doses can cause chromosomal damage that may not be apparent immediately. The majority of chromosomal damage results
from the process of indirect action of ionizing radiation
on vital biologic macromolecules.
Almost every type of chromosome aberration can be
caused by exposure to ionizing radiation. However,
some aberrations can “only” be produced by radiation
exposure.5 The total radiation dose delivered to a somatic or genetic cell and the duration in which the dose
was delivered determine the rate of production of chromosome aberrations.
Attempts have been made to measure chromosome
aberrations after diagnostic x-ray imaging procedures,
but successful results have not been achieved in these
studies. For imaging procedures that involve much higher
radiation dose rates, studies demonstrated that radiationinduced chromosome imperfections were observed
shortly after the imaging procedure was completed.
Increased frequency of chromosome translocations
is an established radiation biomarker and may also
A
1
C
6
D
13
F
19
2
8
7
14
20 21 22
15
G
B
3
9
E
4
10
16
11
17 18
Sex
X Y
chromosomes
5
12
suggest increased cancer risk.11 An occupational epidemiologic study of 146,000 US radiologic technologists began in 1982 and is still in progress. This
study is a collaborative effort of the University of
Minnesota School of Public Health, the National Cancer Institute, and the American Registry of Radiologic
Technologists.
12,13
The purpose of the research is “to
determine whether their personal cumulative exposure to diagnostic x-rays was associated with increased frequencies of chromosome translocations”11
and possible associated cancer risk. Included in the
study were mail surveys, telephone interviews, and a
collection of 150 blood samples for testing purposes.
Results of the blood tests indicated increased chromosome damage as a consequence of cumulative workrelated exposure from routine x-ray procedures.11 For
patients, computed tomography (CT) and nuclear
medicine procedures can contribute substantially to
higher radiation exposure. “Some studies have found
increased chromosome abnormalities immediately
after radiation exposure from CT scanning”
11,14
or in
patients with unusually high numbers of diagnostic
procedures.
11,15
These studies, however, have failed to
show similar levels for low dose CT versions of the
same scan.
150
S U M M A R Y
CHAPTER 8 Early Tissue Reactions and Their Effects on Organ Systems
• Biologic effects that occur relatively soon after humans receive high doses of ionizing radiation are
generally referred to as early effects.
• Early tissue reactions are not common in diagnostic
radiology.
• Somatic effects are effects upon the body that was
irradiated.
• Genetic effects are effects upon future generations due
to the irradiation of germ cells in previous generations.
• Somatic tissue reactions include cell killing and are
directly related to the dose of radiation received. As
dose increases, so does the severity of these early
tissue reactions.
• Early tissue reactions vary depending on the duration
of time after exposure to ionizing radiation. They
may appear within minutes, hours, days, or weeks
after receiving a high dose of ionizing radiation.
• Possible high radiation dose consequences generally
include nausea and fever, extreme fatigue, erythema,
epilation, and blood and intestinal disorders. Also,
temporary or permanent sterility in the male and
female and injury to the central nervous system (at
extremely high radiation doses) can occur.
• Acute radiation syndrome (ARS) occurs in humans
after large whole-body doses of ionizing radiation
delivered over a short period.
• ARS can manifest as hematopoietic syndrome,
gastrointestinal syndrome, and cerebrovascular
syndrome.
• ARS presents in four major response stages: pro-
dromal, latent period, manifest illness, and recovery or death.
• LD (lethal dose) 50/30 signifies the whole-body dose
of ionizing radiation that can be lethal to 50% of an
exposed population within 30 days.
• LD 50/30 for adult humans is estimated to be 3 to
4 Gyt without medical support.
• When cells are exposed to sub-lethal doses of ion-
izing radiation, repair and recovery are possible.
• After receiving a sub-lethal dose of radiation,
surviving cells will be able to divide and thereby
begin to repopulate in the irradiated region.
• Approximately 90% of radiation-induced damage
may be repaired over time; 10% is irreparable.
• High radiation doses to any part of the human body
can result in local tissue damage.
• Significant cell death usually results after substantial
radiation exposure, leading to potential atrophy of
involved organs and tissues.
• Depending on the types of cells included and the
dose of radiation received, recovery may be partial or
complete, or it may not occur, resulting in failure of
the irradiated biological structure.
• Factors such as radiosensitivity, reproductive characteristics, and growth rate govern organ and tissue
response to radiation exposure.
• Many early radiologists and dentists developed radiodermatitis as a consequence of radiation exposure
to the skin that eventually led to the development of
cancerous lesions.
• Human skin consists of three layers and several
accessory structures, all of which are actively
involved in the response of tissue to radiation
exposure.
• A single absorbed dose of 2 Gyt can cause radia-
tion-induced skin erythema within 24 to 48 hours
after irradiation.
• High radiation doses to the skin can cause moist
skin pealing, and then dry desquamation.
• Moderate radiation doses to the scalp can cause
temporary hair loss, and large radiation doses can
result in permanent hair loss.
• Significant evidence of skin damage as a conse-
quence of exposure to orthovoltage radiation
therapy comes from oncology patients who underwent such treatments in earlier years for deepseated tumors.
• The use of high-level fluoroscopy for extended
periods can result in radiation-induced skin injuries for patients.
• Human germ cells are relatively radiosensitive.
• In males, a radiation dose to the gonads of 0.1 Gyt
can depress the sperm population and possibly
cause genetic mutations in future generations.
• In females, a gonadal dose of 0.1 Gyt may delay or
suppress menstruation.
• Personnel dosimeters have replaced periodic blood
counts as an accurate means to monitor occupational radiation exposure.
• When blood counts were used to monitor the effects
of radiation exposure among those who worked with
radiation, a whole-body radiation dose of 0.25 Gyt
CHAPTER 8 Early Tissue Reactions and Their Effects on Organ Systems
151
would be required to produce a measurable hematologic depression.
• The mapping of chromosomes is karyotyping.
• Karyotyping is performed during metaphase, when
each chromosome can be individually perceived
and radiation-induced chromosome and chromatid aberrations can be observed.
• Chromosomal damage can be caused by both low
• Chromosome aberrations have been observed in
G E N E R A L D I S C U S S I O N Q U E S T I O N S
1. Although early tissue reactions are not common in
diagnostic radiology, what type of diagnostic imaging procedure could possibly produce a radiation
dose sufficient to cause such a reaction?
2. In what do unacceptable high x-ray exposures in
radiology primarily result?
3. How can a cytogenetic analysis of chromosomes be
accomplished?
4. What are genetic effects?
5. How have scientists become aware of radiation-
induced skin damage in early pioneers?
6. What are the three separate dose-related syndromes
that occur as part of acute radiation syndrome, and
what are the four major response stages?
7. What is radiodermatitis?
8. After the reception of a single absorbed dose of
9. How has information on the gonadal response to
10. How significant is the chance of causing sterility in
11. What are the somatic effects of radiation exposure?
12. From where does the term somatic originate?
and high radiation doses.
individuals after completion of some imaging
procedures.
2 Gyt of radiation, approximately how long will
it take to cause radiation-induced skin erythema?
radiation exposure been acquired?
imaging personnel who perform routine procedures?
R E V I E W Q U E S T I O N S
1. The total radiation dose given to a somatic or genetic
cell and the period of time in which that dose was
delivered determine the rate of production of:
A. Cell division
B. Chromosome aberrations
C. Genetic analysis
D. Karyotyping
2. Acute radiation syndrome presents in four major
response stages. In what order do these stages occur?
A. Latent period, prodromal, manifest illness, recovery
or death
B. Manifest illness, prodromal, latent period, recovery
or death
C. Prodromal, latent period, manifest illness, recovery
or death
D. Manifest illness, latent period, prodromal, recovery
or death
3. Which of the following systems is the most radiosen-
sitive vital organ system in human beings?
A. Cerebrovascular
B. Gastrointestinal
C. Hematopoietic
D. Skeletal
4. When cells are exposed to sub-lethal doses of ioniz-
ing radiation, approximately _____ of radiationinduced damage may be repaired over time and
about ______ is irreparable.
A. 25%, 75%
B. 50%, 50%
C. 75%, 25%
D. 90%, 10%
5. As radiation dose increases, the severity of early
tissue reactions:
A. Also increases
B. Gradually decreases
C. Increases sharply and then gradually decreases
D. Remains constant
6. Prolonged exposure to x-rays in the diagnostic en-
ergy range results in high radiation dose to the skin
while underlying tissues receive:
A. A much greater dose
B. A slightly greater dose
152
CHAPTER 8 Early Tissue Reactions and Their Effects on Organ Systems
C. No dose
D. A substantially less dose
7. In 1898 after personally developing burns attributed
to radiation exposure, this Boston dentist began investigating the hazards of radiation exposure and
became the first advocate of radiation protection.
Who is this person?
A. William Herbert Rollins
B. Wilhelm Conrad Roentgen
C. Thomas Alva Edison
D. Clarence Madison Dally
8. In the female, the ovarian stem cells:
A. Begin as a single cell during fetal development,
before birth, and then gradually increase in number throughout life
B. Multiply to a few hundred cells during fetal life,
before birth, and then gradually increase in number throughout life
C. Multiply to millions of cells only during fetal de-
velopment, before birth, and then steadily decline
in number throughout life
D. Multiply to millions of cells only during fetal
development, before birth, and then steadily
continue to increase in number throughout life
9. Which of the following types of cells develop from
single precursor cell, the pluripotential stem cell?
1. Lymphocytes and granulocytes
2. Thrombocytes and erythrocytes
3. Platelets
A. 1 only
B. 2 only
C. 3 only
D. 1, 2, and 3
10. With regard to radiation exposure, which part of
the gastrointestinal tract is most severely affected?
A. Esophagus
B. Stomach
C. Small intestine
D. Large intestine
Stochastic Effects and Late Tissue
Reactions of Radiation in Organ Systems
O B J E C T I V E S
After completing this chapter, the reader will be able to
perform the following:
• Define all key terms.
• Explain how scientists use epidemiologic studies to
predict the risk of cancer in human populations exposed to low doses of ionizing radiation.
• Explain the purpose of a radiation dose–response
curve.
• Draw diagrams demonstrating various dose–response
relationships.
• Explain the reason regulatory agencies continue to
use the linear dose–response model for establishing
radiation protection standards.
• Differentiate between the threshold and nonthreshold relationships.
9
• List and describe the various late tissue reactions and
stochastic effects of ionizing radiation on living systems.
• Describe the concept of risk for radiation-induced
malignancies, and explain the models that are used
to provide risk estimates.
• Identify ionizing radiation-exposed human populations or groups that prove radiation induces cancer.
• Explain how spontaneous mutations occur.
• Discuss the concept and processes of radiationinduced genetic effects.
• Differentiate between dominant and recessive gene
mutations.
• Explain the doubling dose concept, and provide an
example of how the number of mutations increases
as dose increases.
C H A P T E R O U T L I N E
Epidemiology
Carcinogenesis
Radiation Dose–Response Relationship
Dose–Response Curves
Threshold and Nonthreshold Relationships
Risk Models Used to Predict Cancer Risk and
Heritable Damage in Human Populations
Risk Models Used to Predict Leukemia, Breast
Cancer, and Heritable Damage
Risk Model Used to Predict High-Dose Cellular
Response
The Rationale for Risk Model Selections
Somatic Effects
Late Somatic Effects
Risk Estimate for Contracting Cancer From
Low-Level Radiation Exposure
Low-Level Effects Summary
Major Types of Late Effects
Risk Estimates for Cancer
Life Span Shortening
Cataractogenesis
Embryologic Effects (Birth Defects)
Genetic (Hereditary) Effects
Irradiation Mutations
Natural Mutations
Other Agents of Genetic Mutations
Incapacities of Mutant Genes
Dominant or Recessive Point Mutations
Ionizing Radiation as a Possible Cause of Genetic
(Hereditary) Effects
Doubling Dose Concept
Summary
153
154
CHAPTER 9 Stochastic Effects and Late Tissue Reactions of Radiation
K E Y T E R M S
absolute risk
carcinogenesis
cataractogenesis
doubling dose
embryologic effects (birth
defects)
epidemiology
genetic, or hereditary, effects
late somatic effects
late tissue reactions
linear nonthreshold curve
linear-quadratic nonthreshold
curve
nonthreshold
organogenesis
radiation dose-response
relationship
relative risk
sigmoid, or S-shaped (nonlinear)
threshold curve
stochastic effects
threshold
Radiation-induced damage at the cellular level may lead
to measurable somatic and hereditary damage in the
living organism as a whole later in life. These late effects
are the long-term results of radiation exposure. Some
examples of measurable delayed biologic damage are:
• Cataracts
• Leukemia
• Genetic mutations
Cataracts are considered to be a late tissue reaction
that is nonrandom, whereas leukemia and genetic mutations are viewed as delayed stochastic or random
consequences that, if these reactions do appear, they do
not do so for extended periods. This chapter focuses
on the organic system-level damage from ionizing
radiation that occurs months or years after radiation
exposure.
EPIDEMIOLOGY
Epidemiology is a “science that deals with the inci-
dence, distribution, and control of disease in a population.”1 Epidemiologic studies consist of observations
and statistical analysis of data, such as the incidence of
disease within groups of people. The latter studies include the risk of radiation-induced cancer. The incident
rates at which these irradiation-related malignancies
occur are determined by comparing the natural incidence of cancer occurring in a human population with
the prevalence of cancer occurring in an irradiated
population. Risk factors are then identified for the
general human population.
Epidemiologic studies are of significant value to ra-
diobiologists who use the information from these studies to formulate dose–response estimates for predicting
the risk of cancer in human populations exposed to low
doses of ionizing radiation.
CARCINOGENESIS
Carcinogenesis, also called tumorigenesis, is the formation of a cancer. Cancer is the name used for a substantial group of diseases in which healthy cells have
been transformed into nonstandard cells that divide
uncontrollably. The process leads to an expansive
growth of abnormal structures within various locations in the body and the destruction of surrounding
body tissues such as bone marrow. The altered or cancer cells readily demonstrate the potential to invade or
spread to other parts of the body. Cancer is the most
significant late stochastic effect caused by exposure to
ionizing radiation.
RADIATION DOSE–RESPONSE RELATIONSHIP
Dose–Response Curves
The radiation dose–response relationship is demon-
strated graphically through a curve (the dose–response
[DR] curve) that maps the observed effects of radiation
exposure in relation to the dose of radiation received.
The “effect” in question may be the incidence of a disease (e.g., cases of cancer per million in a population or
fatalities due to cancer per million in a population), or
the effect may be its degree of acuteness, such as the
severity of cataracts as dose increases. The DR curve is
either linear (straight line) or nonlinear (curved to
some degree), and it depicts either a threshold dose or
a nonthreshold dose (Fig. 9.1).
Threshold and Nonthreshold Relationships
The term threshold is defined as a point or level at
which a response or reaction to an increasing stimulation first occurs. Regarding ionizing radiation, this
CHAPTER 9 Stochastic Effects and Late Tissue Reactions of Radiation
Response to radiation
1
155
Response to radiation
(expressed in biologic effects observed)
A
Response to radiation
(expressed in biologic effects observed)
B
Fig. 9.1 (A) 1 represents a linear (straight-line) nonthreshold
curve of radiation dose–response relationship; 2 represents a
linear threshold curve of radiation dose–response relationship;
3 represents a nonlinear threshold curve of radiation dose–
response relationship. (B) Sigmoid (S-shaped, hence nonlinear)
threshold curve of radiation dose–response relationship generally employed in radiation therapy to demonstrate high-dose
cellular response.
Radiation dose received
(Threshold)
Radiation dose received
2
3
(expressed in biologic effects)
Radiation dose
Fig. 9.2 Linear nonthreshold curve of radiation dose–response
relationship. The straight-line curve passing through the origin
in this graph indicates both that the response to radiation (in
terms of biologic effects) is directly proportional to the dose
of radiation and that no known level of radiation dose exists
below which the chance of sustaining biologic damage is zero.
In contrast to a cell-survival curve (Fig. 7.17), as seen in Chapter
7, both the vertical and horizontal axes of a dose–response
curve are ordinary linear scales.
means that below a certain absorbed radiation dose, no
biologic effects are observed. The biologic effects begin
to occur only when the threshold dose is reached. A
nonthreshold relationship indicates that a radiation
absorbed dose of any magnitude has the capability of
producing a biologic effect. Therefore, if the DR curve is
as shown in Fig. 9.2, biologic effect responses will be
caused by ionizing radiation in living organisms in a
directly proportional manner at any dose above zero.
This behavior is referred to as a linear nonthreshold
(LNT) relationship. LNT proclaims that no radiation
dose can be considered absolutely “safe,” with the incidence of the biologic effects increasing directly with the
magnitude of the absorbed dose.
Risk Models Used to Predict Cancer Risk and Heritable Damage in Human Populations
In a 1980 report, the Committee on the Biological Effects of Ionizing Radiation (BEIR), under the auspices
of the National Academy of Sciences, studying atomic
bomb survivors concluded that most stochastic effects
(e.g., cancer) and hereditary effects at low-dose levels
156
Response to radiation
Response to radiation
(expressed in biologic effects)
Fig. 9.3 Linear-quadratic nonthreshold dose–response relation-
ship. The curve estimates the risk associated with low-dose
levels from low LET radiation.
CHAPTER 9 Stochastic Effects and Late Tissue Reactions of Radiation
c
Linear
e
n
i
L
Radiation dose
i
t
a
r
d
a
u
q
-
r
a
from low LET radiation, such as the type of radiation
used in diagnostic radiology, appear to follow a linearquadratic nonthreshold dose–response curve (LQNT
DR) (Fig. 9.3). The term linear-quadratic implies that
the equation that best fits the data has components that
depend on dose to the first power (linear or straight-line
behavior) and also on dose squared (quadratic or curved
behavior). Since the 1980 report, newer risk models and
updated dosimetry techniques have provided a more
useful follow-up study of Hiroshima and Nagasaki
atomic bomb survivors. In 1990 the BEIR Committee’s
revised risk estimates indicated that the risk from radiation exposure was about three to four times greater than
previously projected. Currently, the committee recommends the use of the linear nonthreshold curve of radiation dose–response (LNT DR) for most types of
cancers. With the LNT DR curve, if the absorbed dose is
doubled, the biologic response probability, and therefore its actual occurrence in a large population sample,
is also doubled (see Fig. 9.2).
humans are exposed to radiation during diagnostic imaging procedures. Another nonthreshold risk estimate
curve is the LQNT DR curve (see Fig. 9.3). This curve
displays a more conservative dose–response outcome for
low-level radiation. The 1990 BEIR Committee considered the LQNT relationship to be an improved reflection
of stochastic and genetic effects at low-dose levels from
low-LET radiation. The following health concerns are
presumed to follow this DR curve:
• Leukemia
• Breast cancer
• Heritable damage
For leukemia, the LQNT hypothesis appears to be
supported by an analysis of the leukemia occurrences in
Nagasaki and Hiroshima that utilized a more recent reevaluation of the radiation dose distribution in these
two cities.
2,3
Risk Model Used to Predict High-Dose Cellular Response
Acute reactions from significant radiation exposure,
such as skin erythema and hematologic depression, may
be demonstrated graphically through the use of a radiation linear threshold dose–response curve (LT DR) as
shown in Fig. 9.4. In this model, a biologic response
does not occur below a specific dose level. Laboratory
(Linear)
(expressed in biologic effects)
Risk Models Used to Predict Leukemia, Breast Cancer, and Heritable Damage
Currently, advocates of LNT theorize that because
all radiation exposure levels possess the potential to
cause biologic damage, radiographers must never fail to
employ aggressive radiation safety measures whenever
Threshold
Radiation dose
dose
Fig. 9.4 Linear threshold curve of radiation dose–response.
This depicts those cases for which a biologic response does
not occur below a specific radiation dose.