Download Radiation Quantities and Units

Long-Term Somatic Effects
of Radiation
In This Lecture
Somatic Cell Mutations and Cancer
Radiation Induced Cancer in Humans
Extrapolating to Low Doses
Radiation Induced Cataracts
TYPE OF RADIATION DAMAGE
The effects of radiation on the human population can
be classified as either somatic or genetic:
o Somatic effects are harm that exposed individuals
suffer during their lifetime, such as radiation induced
cancers (carcinogenesis), sterility, opacification of the
eye lens and life shortening.
o Genetic or hereditary effects are radiation induced
mutations to an individual’s genes and DNA that can
contribute to the birth of defective descendants.
Long-Term Somatic Effects
Somatic cells are those cells in your body other
than reproductive cells . They can be damaged in
a variety of ways, such as by chemicals, biological
and physical agents or by ionizing radiation . The
effects of the damage from ionizing radiation can
be long-term or short-term depending on the
type and severity of the damaging agent.
The long-term effects of radiation exposure are
cancer and cataract formation.
Somatic and Genetic Effects
Carcinogenesis expresses itself as a late somatic effect.
Sources of human data on carcinogenesis:
– Low level occupational exposure.
– Atomic bomb survivors in Hiroshima and Nagasaki.
– Medical radiation exposure of patient:
• treatment of ankylosing spondylitis with orthovoltage x rays
• treatment of thyroid abnormalities with radiation
• radiotherapy in cancer treatment
– Exposure of staff during medical procedures
• Early radiologists
• Early radiation oncologists using brachytherapy
Somatic Cell Mutations and Cancer
• A long-term somatic effect is the damage to cells that are
continually reproducing. These cells are the most sensitive
to radiation because any changes made in the parent cell's
chromosome structure will be transmitted to its daughters.
Also, radiation can affect the delicate chemistry of the cell
causing changes in the rate of cell division or even the
destruction of that cell.
• An event which causes a somatic cell to behave in this way
is called a mutation . Mutations in the reproductive
cells cause damage that affects future generations. However,
a mutation in a somatic cell has consequences only for the
individual.
Somatic Cell Mutations and Cancer
• If the mutation in the somatic cell increases the
rate of its reproduction in an uncontrolled
manner, then the number of daughter cells may
increase rapidly in that area. When this occurs, it
often happens that the daughter cells divide
before reaching their mature state. The result
then is an ever increasing number of cells that
have no beneficial function to the body, yet are
absorbing body nutrition at an increasing rate.
• The tissue could now be called a tumor.
Somatic Cell Mutations and Cancer
Benign tumor :cells remain in their place of origin
and do not directly invade surrounding tissues.
Malignant tumor : tumor invades neighboring tissue
and causes distant secondary growths (called
metastasis ).
• A malignant tumor is what we call “cancer”, whether
it is fatal or not depends on:
– The tissue in which it is located
– How rapidly it grows
– How soon it is detected and treated
Radiation Induced Cancer in Humans
 Leukemia :is a disease characterized by a great excess of
immature white cells in the blood and can be likened to a
blood cancer. Marie Curie , for example, who first
isolated radium from uranium ore died of leukemia as did
her daughter- assistant, (her husband on the other hand
died in a traffic accident).
 In the 1920's, watch dials were painted with a radium
based luminous paint. The employees, all women, who
did this work often licked their paint brushes to give
them a sharp point and ingested a small quantity of the
paint each time they did this. The radium in the paint
collected in the bones of these employees and resulted
in bone tumors 8 to 40 years later.
Radiation Induced Cancer in Humans
The distribution of cancers among the 1346 radium dial painters
that were followed up. It demonstrates quite clearly that:
The risk of cancer increases with the radiation dose.
Radiation Induced Cancer in Humans
In UK 6500 patients with arthritis of the spine were treated with large doses of xrays . The average dose was 3Gy. The disease called ankylosing spondylitis , causes a
painful stiffening of the joints in the backbone.
Of the 6500 patients, 30 developed leukemia compared with an expected incidence
of 7 cases.
The survivors of the Hiroshima and Nagasaki atom bomb attacks. Nearly 80,000 of
these people have been carefully studied in the years since the war. Of this number
of survivors, 126 died of leukemia. This provided clear evidence of the dose
dependent relationship of leukemia to radiation, i.e.:
The higher the dose, the greater the risk.
The study of these survivors also indicated an increase in frequency of stomach ,
lung and breast cancers . These have taken much longer to develop and some are
still appearing now.
The A-bomb data from Japan shows that that there is a delay (called the latency )
between the radiation exposure and the death from cancer it induces. The mean
latency from leukemia is in the region of ten years, and for the other cancers it is
more than twenty years. Even now, more than 40 years after the explosions, excess
cancer deaths are still occurring.
Extrapolating to Low Doses
There is no doubt that various forms of cancer are
produced in people who have received large doses (of
up to several Gray) in short periods of time.
It is known that for low LET radiation (x-rays , rays and beta rays ) exposures received over a long
period of time (such as a year) have less effect than if
received over a period of a few hours. For high LET
radiation the opposite appears to be true. Similarly the
biological effect is not strictly proportional to the
received dose and smaller doses become less and less
efficient in producing an effect.
Extrapolating to Low Doses
The body does not contain any organs whose specific function is to undo
radiation damage although it is known that individual cells do have the
ability to repair some types of damage.
The question of whether a threshold radiation dose for cancer exists is a
subject of some debate in scientific circles. Because of the lack of definitive
data it is at present assumed that:
There is no radiation dose which can be said to be perfectly safe.
There may be a threshold dose for cancer but its existence has not been
demonstrated, and therefore it is prudent to assume that even the smallest
dose has the possibility of producing a harmful effect (albeit with a very
small probability). It is for this reason that:
All unnecessary exposure should be avoided.
Effects such as cancer induction and genetic damage are referred to as
stochastic effects , because the initiating event occurs randomly and there is
no definite threshold. The severity of the effect also does not depend on
the dose, but rather the probability that the effect occurs increases with
increasing dose.
Extrapolating to Low Doses
Nominal probability coefficients for stochastic effects
Determent ( % per Sv)
Exposed population
Fatal
cancer
Adult workers
Whole Population
4.0
5.0
Severe
Non-fatal
hereditary
cancer
effects
0.8
0.8
1.0
1.3
Total
5.6
7.3
This means that if your whole body is exposed to 1 Sv of ionizing
radiation , you have an extra 4% chance of contracting a
cancer that will be fatal many years after the exposure. The
word ‘extra’ is used because you normally have a 20 to 25%
chance of dying from cancer.
Radiation Induced Cataracts
Cataract : used to describe a loss of transparency of the lens of
the eye . Cataracts are commonly associated with aging and also
with metabolic conditions such as diabetes.
The lens of the eye is peculiar in that there is no cell
replacement system and therefore damaged cells that have
become opaque are not replaced naturally.
Unlike the induction of cancer, radiation damage to the lens
of the eye shows a definite threshold effect. As long as the lifetime equivalent dose is less than 7.5 Sv , no lens opacities due to
ionizing radiation will be produced that will interfere with vision.
Radiation effects that show a definite threshold are called
deterministic effects . Deterministic effects do not occur below
the threshold, but above the threshold:
The severity of the effect increases with increasing dose.
Radiation Induced Cataracts
The Degree of Opacity:
1+
3+
2+
4+