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_5255_Библиотеки_им_академика_М_И_Перельмана
.pdf
482
Basics Concepts in Radiobiology
c) Oxygen tension in tissues (Oxygenation Effect): The impact of low- versus highLET radiation can be observed in hypoxic tumour cells. The action of radiation in an
absorbing medium is the production of ion pairs and free radicals. Oxygen free radicals are
highly reactive molecules owing to their unpaired valence electron, and they play a role in
the breaking of chemical bonds. The effect of this free radical-mediated injury is greatly
influenced (enhanced) by the presence of oxygen. When tumour cells rapidly divide, they
outgrow their vascular supply and become hypoxic. With increase in LET, the dependence
on oxygenation for biological effect decreases and oxygen has a negligible effect in highLET radiation such as alpha particles or heavy ions. However, low-LET radiation sources
such as x-rays and gamma rays rely significantly on oxygen for radiation effects.
Oxygen Enhancement Ratio (OER): The dose modification under the influence of
oxygen is expressed by a factor known as oxygen enhancement ratio (OER). This parameter
is defined as the ratio of radiation dose required for producing a given biological effect
under hypoxic condition to that under well aerated conditions. The partial pressure of
oxygen in atmospheric air is around 20% of 760mm (or 150 mm) Hg. At cellular level in
human body, this pressure is about 15-20mm Hg. The oxygen supply to the tumour cells is
through the vascular structures. In solid tumours the oxygen from the capillary may diffuse
through the stroma over a distance of about 150 to 200 m beyond which the oxygen
pressure falls and cells survive under hypoxic or even in anoxic condition. It is now well
known that hypoxic cells are less sensitive to radiation and cells with complete anoxia are
radioresistant. For low LET radiation OER has a value ranging from 2.5 - 3. At elevated
temperatures, however, hypoxia does not protect cells against radiation damage and rather
augments the effect due to low pH (hyperthermia). The combination of radiation with heat
therefore looks to be an attractive modality of treating cancer cells. The aerated cells are
sensitive to radiation whereas the hypoxic cells are more sensitive to treatment at elevated
temperature.
Relationship of RBE and OER with LET
The increase in LET enhances RBE as expected but only up to about 100 keV/m beyond
which RBE starts falling due to overkill effect (Figure 1). As LET increases the value of
OER falls because cell killing is mostly by single track events.
Basics Concepts in Radiobiology
483
Figure 1: The effect of LET on RBE and OER
Radiosensitivity
The radiosensitivity of a biological system depends primarily on the stage of the cell cycle
and oxygen tension present at the time of irradiation. The cell cycle may be divided into
three phases; S-phase (the DNA synthesis), G1 and G2 phase (pre and post synthesis gaps)
and mitosis (M). Cells are more sensitive to radiation in G2 and M phase. The sensitivity in
various stages of the cell cycle can be given in sequence as M>G2>S>G
Cells in culture
1.
can be blocked at G2-M boundary so as to allow all the dividing cells to pile up at this stage
and then released to undergo mitosis before being exposed to radiation. This can optimise
the radiosensitivity. Such blocking is possible in in vitro experiments but difficult to translate
to in vivo conditions. J. Bergonie and L. Tribondeau formulated a law as early as in 1959
that the radiosensitivity of a tissue is proportional to the proliferating activity and inversely
proportional to the degree of differentiation of its cells (3). Chemical modifiers are also
available which are known to increase the radio-sensitivity (sensitizers) of a system. The
best radiosensitizer known so far is oxygen as cells are found to be more sensitive to
radiation in a well oxygenated condition than in hypoxia.
Cellular Sensitivity to Radiation
Not all living cells are equally sensitive to radiation; those with active proliferation are more
484
Basics Concepts in Radiobiology
sensitive. As a result, living cells can be classified according to their rate of reproduction,
which also indicates their relative sensitivity to radiation. This means that different cell
systems have different sensitivities. Lymphocytes (white blood cells) and cells, which produce
blood, are constantly regenerating, and are, therefore, the most sensitive. Reproductive and
gastrointestinal cells are not regenerating as quickly and are relatively less sensitive. The
nerve and muscle cells are the slowest to regenerate and are the least sensitive cells.
Cells, like the human body, have a tremendous ability to repair damage. As a result, not
all radiation effects are irreversible. In many instances, the cells are able to completely
repair any damage and function normally. If the damage is severe enough, the affected cell
dies. In some instances, the cell is damaged but is still able to reproduce. The daughter cells,
however, may be lacking in some critical life-sustaining component, and ultimately die. The
other possible result of radiation exposure is that the cell is affected in such a way that it
does not die but is simply mutated. The mutated cell reproduces and perpetuates the mutation,
which could even lead to the transformation to malignancy.
The sensitivity of the various organs of the human body correlate with the relative
sensitivity of the cells from which they are formed. For example, the blood forming cells are
one of the most sensitive cells due to their rapid regeneration rate therefore the blood
forming organs are most sensitive organs to radiation. Muscle and nerve cells are relatively
insensitive to radiation, so are the muscles and the brain.
The rate of reproduction of the cells in an organ system is not the only criterion of
determining overall sensitivity. The relative importance of the organ system to the well
being of the body is also important. The outer cell layer in a tumor has a good supply of
blood and oxygen and reproduces rapidly and is sensitive to radiation. Anoxic cells located
in the interior core of a tumor tend to be inactive and are radioresistant. As the tumor is
exposed to radiation, the outer layer of rapidly dividing cells is destroyed, causing it to
“shrink” in size. With a small dose each day, giving healthy tissue a chance to recover from
any damage gradual shrinking of sensitive tumor takes place. The cell system that is composed
of rapidly dividing cells with a good blood supply and lots of oxygen is the developing
embryo. Therefore, the sensitivity of the developing embryo to radiation exposure is very
high and needs proper protection against radiation.
Target theory and survival curves
The target theory is a mathematical model, which calculates the fraction of cells in a system
that survives a given dose of radiation. In its simplest form the theory assumes that a single
hit (ionisation) is sufficient to produce the measured effect. The measured effect may be the
cell death or its inability to divide. This form of target theory is over simplified and
therefore modified versions of the theory are needed to explain the radiation damage in
complex systems. The basic concept is explained here.
Basics Concepts in Radiobiology
485
Simple target theory
According to this model one hit is sufficient to inactivate one target. At low dose range the
effect is proportional to radiation dose because only a small part of the total number of
targets is damaged. With an increase in the dose the probability of a hit in the same target
increases thereby reducing its effectiveness. This over killing effect at higher doses makes
the net biological effect non-linear. The surviving fraction of cell population falls
exponentially with increase in radiation dose. Thus the same increment in dose does not kill
the same number of cells but will damage the same proportion of cells that have survived
until then as can be seen in figure 2a.
Figure 2a: Relation between dose of radiation and fraction of cells surviving
The mathematical formulation of the dose effect relationship can be considered by
assuming No as the initial number of cells in an organism and N as the number that survives
a dose D. Let a small increment in dose (dD) inactivates dN number of cells. Then
mathematically,
dDdN
NdN
dDNdN
dDkNdN
Where k is a constant. The negative sign shows that the number of targets decreases as
dose increases. On integration equation -1 takes the following form:
kDkD
eNNeNN
or
00
(1)
(2)
486
Basics Concepts in Radiobiology
or the surviving fraction S =
where
1 Dk
0
NN
o
DDkD
0
eeS
(3)
Let us define Do as the dose that produces, on the average, one hit per target.
At
DD , equation-3 may be written as:
0
1
or S = 1/2.71 = 0.37 (4)
eS
Hence at a dose Do, 37% of the cells survive and 63% of them die. The quantity Do is
the measure of radiosensitivity of cells and can be determined from survival curves as the
dose at which about 37% of the initial number of cells survive. The survival curve has an
exponential form and becomes linear on a semi-logarithmic scale in figure 2b. The dose D
is also termed as D
equal but the same is not true for the shouldered portion
For the exponential part of the survival curve both Do and D37 are
37.
.
o
Figure 2b: Simple form of survival curve plotted in a semi log display
Practically most of the survival curves have some initial shoulder (Figure 3), which
cannot be explained by simple exponential form. The terminal straight portion may be
extrapolated back and the point where it meets the ordinate is called the extrapolation
Basics Concepts in Radiobiology
487
number (n). Dq (quasi-threshold dose) is a measure of shoulder width, the dose at which the
straight portion of the survival curve, extrapolated backward, cuts the dose axis drawn
through a survival fraction of unity. The shoulder on a survival curve determines the ability
of cells to restore their viability by recovery of sub-lethal damage. This ability of cells
diminishes beyond certain doses and the curve assumes an exponential form. The slope of
the exponential part is a good measure of radiosensitivity of the system studied.
Figure 3: Survival curve in semi-logarithmic scale showing quasi-threshold dose and extrapolation
number ‘n’.
Various models of the target theory were proposed e.g. single hit multi target, multi hit
multi target etc. but the observed form of the survival curve could not be explained fully by
any one of them. The linear quadratic model which splits the survival curves into linear and
quadratic components appears to be more attractive than others (Figure 4). The linearquadratic model assumes that there are two components to cell killing by radiation, one that
is proportional to dose and one that is proportional to the square of the dose. The linear
and quadratic contributions to cell killing are equal at a dose that is equal to the ratio of to
.
488
2
( )
D D
Basics Concepts in Radiobiology
Figure 4: Representation of survival curve for high and low LET radiation (graph not to scale).
The LQ model shows the change in the value of / with increase in radiation dose.
The linear quadratic model
The effects of radiation in most of the biological systems follow, what is known as, the
linear quadratic model. The average frequency of lethal events at dose D may be taken as
D + D2. Hence the surviving fraction to a dose D can be given by the following formula:
S e
The coefficients and relate to two modes of death and their ratio is a parameter
which shows their relative importance. The effect follows a linear whereas effect follows
a quadratic dose response relationship. The high LET radiation follow mostly effect
whereas low LET radiation (x and gamma rays) follow effect. The higher value of /
makes the survival curve exponential whereas the shoulder of survival curve is broad for its
smaller values.
These mathematical models have practical applications and are widely used in
experimental and clinical research. The equations have been derived for various models of
survival curves but have their limitations, which must be kept in mind before using them in
clinical practice.
Basics Concepts in Radiobiology
100
( / )
489
Measurement of surviving fraction
The capability of a single cell to grow into a large colony, which can be seen easily with the
naked eye, is a convenient proof that it has retained its reproductive integrity. The loss of
this ability as a function of radiation dose is described by the dose-survival curve. A cell
survival curve describes the relationship between the surviving fraction of cells (clonogenic
cells) and the absorbed dose. Conventionally cell survival as a function of dose is graphically
represented by plotting the surviving fraction on a logarithmic scale on the ordinate against
dose on a linear scale on the abscissa. The standard approach for measuring cell killing
using in vitro cell models has been the clonogenic assay originally developed by Puck and
Marcus in the 1950s (4). This has corroborated experimental studies of radiation cell killing
in cell and tumor models, however it is based on a narrow range of assumptions regarding
the definition of clonogenic survival. The accepted criteria assume that a colony derived
from a single cell consists of clonal descendents of the initial cell, each with the same
proliferative potential as the starting cell.
Actively growing cells are prepared as free cell suspension and number of cells per unit
volume is counted with coulter counter. Normally 100 cells are seeded into a dish using
agar, which settles in the medium, and single cells remain at their respective locations.
Cells are incubated for 1-2 weeks and colonies are counted. The number of colonies formed
out of 100 cells is called plating efficiency (PE). A large number of dishes are prepared and
sets of them are exposed to graded doses of radiation. The surviving fraction can be
determined as:
Surving fraction
colonies formed
cells seeded PE
Bystander effect
In the past with the explanation of “Hit theory” researchers believed that radiation damages
only those cells that are “hit” by direct or indirect action. A major advance in understanding
radiation effects has been the observation that cells can respond when their neighbours are
irradiated, referred to as a bystander response. These responses were first clearly identified
in 1992 by Nagasawa and Little and was named as “Bystander effects”(5). This theory
proposes the idea that radiation causes free radicals to trigger cell-cell communication and
cell-matrix communication to cells other than those which are “hit” by the direct ionisation
by means of direct contact or release of material. Initial radiation induced changes to bystander
cells are very frequent events, suggesting total tissue involvement. Bystander effects indicate
that radiation-induced carcinogenesis, genomic instabilities are not a single cell event, but a
tissue and organ response.
490
Basics Concepts in Radiobiology
Adaptive responses
Adaptive responses have challenged traditional thinking in radiation effects. These are
observed when a small dose of (priming) radiation reduces the effect of a larger (challenge)
dose, typically given several hours later. The details on this are described in a chapter on
“Radiation Hormesis” in this book.
Effects of whole body irradiation
Whole body exposures in excess of a few mSv are quite unusual in medicine, but the effects
of higher exposures are described here for completeness. Acute radiation exposure may be
so severe in certain unforeseen circumstances that consequences may be fatal. Death is
generally the result of severe cell depletion in one or more vital organ systems in the body.
A lethal dose is often described by its midpoint, the LD
the individuals would be expected to die in 60 days. For healthy adults the LD
acute exposure is estimated to be 4-5 Gy and the cause of death attributed to the loss of
bone marrow function.
(a) Prodromal radiation syndrome: The human prodromal syndrome varies with respect
to the time of onset, maximum severity and duration depending upon the radiation dose
received. With doses of a few thousand cGy, various phases of acute effects can be expected
within 5-15 minutes of exposure. Depending on the dose received the prodromal syndrome
might reach its maximum within 30 minutes to few days. When the intensity of the prodromal
syndrome gradually diminishes, it merges with the universally fatal vascular syndrome or
after a lower dose, with a fatal GI syndrome. The GI symptoms are anorexia, nausea,
vomiting, diarrhea, intestinal cramps, salivation, fluid loss, dehydration and weight loss.
The neuromuscular symptoms include easy fatigue, apathy or restlessness, sweating, fever,
headache and hypotension. All of these signs and symptoms are not seen unless the exposure
is in the supralethal range.
, that is the dose at which 50% of
50/60
50/60
after
(b) Haematopoietic syndrome: At a dose of 3-8 Gy, death may occur as a result of
radiation damage to the haematopoietic system. Mitotic active precursor cells are sterilized
by the radiation, and the subsequent supply of mature white cells, red cells and platelets is
therefore diminished. The time of potential crisis, when the number of circulating cells in
the blood reaches a minimum value, may be delayed for some time. It is only when the
mature circulating cells begin to die off and the supply of new cells from the depleted stem
population is inadequate to replace them, that the full effect of the radiation becomes
apparent.
(c) Gastrointestinal (GI) syndrome: The total body exposure of more than 10 Gy of
gamma rays leads in most mammals to symptoms characteristic of the GI syndrome, resulting
in death after some days (usually within a week). Characteristic symptoms are nausea,
vomiting and prolonged diarrhea. Individuals lose their appetites and appear sluggish and
lethargic. Prolonged diarrhea extending for several days is regarded as a bad sign, because it
Basics Concepts in Radiobiology
491
indicates the absorbed dose to be more than 10 Gy and will prove fatal. After a few days the
individual shows signs of dehydration, loss of weight, emaciation and complete
exhaustion.There is no incidence on record of a human being having survived a dose in
excess of 10 Gy. The symptoms are attributable principally to the removal of the epithelial
lining of the GI tract by radiation.
(d) Cerebrovascular syndrome: The total body dose of the order of 100 Gy of gamma
rays results in death in a matter of few hours. At these doses all organ systems will be
seriously damaged. The GI and hemopoietic systems will be severely damaged and would
fail if the individual lived long enough, but cerebrovascular damage brings death very
quickly, so that the consequences of the other system failures is not important.
Foetal irradiation
Between conception and birth the foetus passes through three basic stages of development:
Pre-implantation (day 1 to 10); Organogenesis (day 11 to 42); Growth stage (day 43 to
birth). Radiation is a known teratogen (causes birth defects). The effects of radiation on the
foetus depend on two factors: the dose and the stage of development at the time of exposure.
The principal effects of radiation on a foetus are foetal or neonatal death, malformations,
growth retardation, congenital defects and cancer induction. Details on this are given in
“radiation safety” under the sub-title “Radiation and pregnancy”
Biological assessment of radiation dose (Biodosimetry)
Biological assessment of radiation exposure (Biodosimetry), is critical in unprecedented
or suspected radiation exposures when physical dosimetry is either not relevant or not
available. It is still necessary to confirm physical dose estimates when available. Accurate
dose estimates are crucial in making life saving medical decisions for exposed persons, for
determining other health consequences, or for reassuring non-exposed persons. Individual
“biomarkers” for a radiation exposure have been sought from the beginning of the nuclear
age. Bender and Gooch made a landmark observation by showing that the frequency of
chromosome aberrations in lymphocytes correlates well with radiation dose (6). The formation
of dicentrics involves an interchange between 2 separate chromosomes, while ring formation
involves a break in the arm of a single chromosome, followed by rejoining to form a ring
and a fragment.
Соседние файлы в папке Библиотека им академика М.И. Перельмана