MKOLOMA S.S

Radiobiology, Dosimetry and Radiation Protection

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Radiation physics for DDR 1

Radiobiology

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Radiobiology (or radiation biology) is the interdisciplinary field of science that studies the biological effects of ionizing and non-ionizing radiation of the whole electromagnetic spectrum, including gamma rays, x-rays, ultraviolet radiation, visible light, microwaves, radio wave, low-frequency radiation (such as used in alternate electric transmission, ultrasound thermal radiation (heat), and related modalities

It is a subset of biophysics

Cell biology

Formerly known as cytology, from the Greek word kytos, "container“

It is a scientific discipline that studies cells their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death This is done both on a microscopic and molecular

level

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Knowing the components of cells and how cells work is fundamental to all biological sciences

Appreciating the similarities and differences between cell types is particularly important to the fields of cell and molecular biology as well as cancer research and developmental biology

These fundamental similarities and differences provide a unifying theme, allowing the principles learned from one cell type to be generalized to other cell types

Therefore, research in cell biology is closely related to genetics, biochemistry, molecular biology, immunology, and developmental biology

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Molecular biology

Is the branch of biology that deals with the molecular basis of biological activity

This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry

Molecular biology chiefly concerns itself with understanding and the interactions between the various systems of a cell, including the interactions between the different types of DNA, RNA and protein biosynthesis as well as learning how these interactions are regulated

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Mechanism of damage by radiation at cellular level

Injury to living tissue results from the transfer of energy to atoms and molecules in the cellular structure

Ionizing radiation causes atoms and molecules to become ionized or excited

These excitations and ionizations can: Produce free radicals. Break chemical bonds. Produce new chemical bonds and cross-linkage between

macromolecules. Damage molecules that regulate vital cell processes

(DNA, RNA, proteins)

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The cell can repair certain levels of cell damage

At low doses, such as that received every day from background radiation, cellular damage is rapidly repaired

At higher levels, cell death resultsAt extremely high doses, cells cannot be

replaced quickly enough, and tissues fail to function

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In general, the radiation sensitivity of a tissue is: proportional to the rate of proliferation of its cells inversely proportional to the degree of cell differentiation

For example, the following tissues and organs are listed from most radiosensitive to least radiosensitive:   Most Sensitive: Blood-forming organs Reproductive organs Skin Bone and teeth Muscle

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Least sensitive: Nervous system

This also means that, a developing embryo is most sensitive to radiation during the early stages of differentiation

And an embryo/fetus is more sensitive to radiation exposure in the first trimester than in later trimesters

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NATURE OF RADIATION DAMAGE AND RBE

Radiation effects can be categorized by when they appear Prompt effects: effects, including radiation sickness

and radiation burns, seen immediately after large doses of radiation delivered over short periods of time.

Delayed effects: effects such as cataract formation and cancer induction that may appear months or years after a radiation exposure

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PROMPT EFFECTS

High doses delivered to the whole body of healthy adults within short periods of time can produce effects such as; blood component changes Fatigue Diarrhea nausea Death

These effects will develop within hours, days or weeks, depending on the size of the dose

The larger the dose, the sooner a given effect will occur

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Partial Body Exposure

These acute effects apply only when the whole body is relatively uniformly irradiated

The effects can be significantly different when only portions of the body or an individual organ system are irradiated, such as might occur during the use of radiation for medical treatment

For example, a dose of 500 rem delivered uniformly to the whole body may cause death while a dose of 500 rem delivered to the skin will only cause hair loss and skin reddening

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DELAYED EFFECTS

Cataracts Cataracts are induced when a dose exceeding

approximately 200-300 rem is delivered to the lens of the eye

Radiation-induced cataracts may take many months to years to appear

Cancer Studies of people exposed to high doses of radiation have

shown that there is a risk of cancer induction associated with high doses

The specific types of cancers associated with radiation exposure include leukemia, multiple myeloma, breast cancer, lung cancer, and skin cancer

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Radiation-induced cancers may take 10 - 15 years or more to appear

There may be a risk of cancer at low doses as wellIt has been difficult to estimate cancer induction risks,

because most of the radiation exposures that humans receive are very close to background levels

At low dose levels of millirems to tens of rems, the risk of radiation-induced cancers is so low

If the risk exists, it is not readily distinguishable from normal levels of cancer occurrence

Leukemia or solid tumors induced by radiation are indistinguishable from those that result from other causes

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The average lifetime risk of death from cancer following an acute dose equivalent to all body organs of 0.1 Sv (10 rem) is estimated to be 0.8%

This increase in lifetime risk is about 4% of the current baseline risk of death due to cancer

The current baseline risk of cancer induction in the United States is approximately 25%

 Another way of stating this risk: A dose of 10 mrem creates a risk of death from cancer

of approximately 1 in 1,000,000

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GENETIC EFFECTS

There is no direct evidence of radiation-induced genetic effects in humans, even at high doses

Various analyses indicate that the rate of genetic disorders produced in humans is expected to be extremely low, on the order of a few disorders per million live born per rem of parental exposure

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PARENTAL RADIATION EXPOSURE

Rapidly proliferating and differentiating tissues are most sensitive to radiation damage

Consequently, radiation exposure can produce developmental problems, particularly in the developing brain, when an embryo/fetus is exposed prenatally

The developmental conditions most commonly associated with prenatal radiation exposure include low birth weight, microcephaly, mental retardation, and other neurological problems

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These effects are related to the developmental stage at which the exposure occurs

The threshold dose for developmental effects is approximately 10 rems

The evidence that the developing embryo/fetus is more sensitive to radiation-induced cancer is inconclusive

But it is prudent to assume that there is some increased sensitivity

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THE RELATIVE BIOLOGICAL EFFCTIVENESS

RBE is a number that expresses the relative amount of damage that a fixed amount of ionizing radiation of a given type can inflict on biological tissues

The higher that number, the more damaging is that type of radiation, for the same amount of absorbed energy

Different types of radiation have different effectiveness mainly because they transfer their energy to the tissue in different ways

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Photons and beta particles have a low linear energy transfer coefficient, meaning that they ionize atoms in the tissue that are spaced

In contrast, alpha particles and neutrons leave a denser trail of ionized atoms in their wake, minimum spaced

The relative biological effectiveness is the radiation weighting factor that enters in the conversion of units of absorbed energy such as rads and grays to units of biological equivalent dose for radiation exposure (such as rems and sieverts, respectively

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Stochastic effects

Are those that occur by chance and consist primarily of cancer and genetic effects

Often show up years after exposure As the dose to an individual increases, the

probability that cancer or a genetic effect will occur also increases

However, at no time, even for high doses, is it certain that cancer or genetic damage will result

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For stochastic effects, there is no threshold dose below which it is relatively certain that an adverse effect cannot occur

In addition, because stochastic effects can occur in individuals that have not been exposed to radiation above background levels, it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure

While it cannot be determined conclusively, it often possible to estimate the probability that radiation exposure will cause a stochastic effect

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It is estimated that the probability of having a cancer in the US rises from 20% for non radiation workers to 21% for persons who work regularly with radiation

The probability for genetic defects is even less likely to increase for workers exposed to radiation

Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur

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Radiation-induced hereditary effects have not been observed in human populations, yet they have been demonstrated in animals

If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation, hereditary effects could occur in the progeny of the individual

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Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants

During certain periods in early pregnancy, may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high

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Nonstochastic effects

Are characterized by a threshold dose below which they do not occur

In other words, nonstochastic effects have a clear relationship between the exposure and the effect

The magnitude of the effect is directly proportional to the size of the dose

Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time

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These effects will often be evident within hours or days

Examples of nonstochastic effects include erythema (skin reddening), skin and tissue burns, cataract formation, sterility, radiation sickness and death

Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (acute & chronic exposure)

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There are a number of cases of radiation burns occurring to the hands or fingers

These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter

Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1,768 R/s

Contact with the source for two seconds would expose the hand of an individual to 3,536 rems, and this does not consider any additional whole body dosage received when approaching the source

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Examples of Nonstochastic effects

Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical

conditions that affect the blood Hemopoietic syndrome conditions appear after a

gamma dose of about 200 rads (2 Gy)

This disease is characterized by depression or ablation of the bone marrow, and the physiological consequences of this damage

The onset of the disease is rather sudden, and is heralded by nausea and vomiting within several hours after the overexposure occurred

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Malaise and fatigue are felt by the victim, but the degree of malaise does not seem to be correlated with the size of the dose

Loss of hair (epilation), which is almost always seen, appears between the second and third week after the exposure

Death may occur within one to two months after exposure

An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow

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Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the

medical conditions that affect the stomach and the intestines

This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater, and is a consequence of the desquamation of the intestinal epithelium

All the signs and symptoms of hemopoietic syndrome are seen, with the addition of severe nausea, vomiting, and diarrhea which begin very soon after exposure

Death within one to two weeks after exposure is the most likely outcome

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Central Nervous System A total body gamma dose in excess of about 2000 rads

(20 Gy) damages the central nervous system, as well as all the other organ systems in the body

Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days

The rapidity of the onset of unconsciousness is directly related to the dose received

In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy), the victim was ataxic and disoriented within 30 seconds

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Other Acute Effects The skin is subject to more radiation exposure,

especially in the case of low energy x-rays and beta rays, than most other tissues

An exposure of about 300 R (77 mC/kg) of low energy (in the diagnostic range) x-rays results in erythema

Higher doses may cause changes in pigmentation, loss of hair, blistering, cell death, and ulceration

Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century.

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The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes

results in temporary sterility among men For women, a 300 rad (3 Gy) dose to the ovaries

produces temporary sterility Higher doses increase the period of temporary sterility In women, temporary sterility is evidenced by a

cessation of menstruation for a period of one month or more, depending on the dose

Irregularities in the menstrual cycle, which suggest functional changes in the reproductive organs, may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization.

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The eyes too, are relatively radiosensitive A local dose of several hundred rads can result in

acute conjunctivitis.

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Carcinogenesis on human beings

A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer

This may be due to the ability to damage or cause disruption of cellular metabolic processes

Several radioactive substances are considered carcinogens, but their carcinogenic activity is attributed to the radiation, for example gamma rays and alpha particles, which they emit

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Common examples of carcinogens are inhaled asbestos, certain dioxins, and tobacco smoke

Cancer is a disease in which damaged cells do not undergo programmed cell death

Carcinogens may increase the risk of cancer by altering cellular metabolism or damaging DNA directly in cells, which interferes with biological processes

It induces the uncontrolled, malignant division, leading to the formation of tumors

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Usually DNA damage, if too severe to repair, leads to programmed cell death

If the programmed cell death pathway is damaged, then the cell cannot prevent itself from becoming a cancer cell

DNA is nucleophilic, therefore soluble carbon electrophiles are carcinogenic, because DNA attacks them

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