The document discusses the effects of radiation on living systems, detailing both direct and indirect injury mechanisms, including DNA damage and free radical formation. It categorizes biological effects into deterministic and stochastic effects, with somatic effects affecting the individual and genetic effects impacting offspring. Additionally, it highlights factors influencing radiation response, such as total dose, tissue type, and age, while also addressing types of cancers associated with radiation exposure.

1. Radiation dose and risk

2. 2
Radiation acts on living systems through
direct and indirect effects.

3. 3
The direct theory of radiation injury
The direct theory of radiation injury suggests that cell damage results when
ionizing radiation directly hits critical areas, or targets, within the cell.
• For example, if x-ray photons directly strike the DNA of a cell,
critical damage occurs, causing injury to the irradiated organism.
• Direct injuries from exposure to ionizing radiation occurs infrequently; most
x-ray photons pass through the cell and cause little or no damage.

4. CHANGES IN DNA
Radiation produces a number of different types of alterations in
DNA, including the following:
BREAKAGE OF ONE OR BOTH DNA STRANDS
CROSS-LINKING OF DNA STRAND TO OTHER DNA STRANDS,
CHANGE OR LOSS OF A BASE

5. 5
Indirect Action:
-xray photons are absorbed by the water within a cell,
free radicals are formed.
These free radicals combine to form toxins.
(e.g.,H 2 O 2 ), which cause cellular dysfunction and
biologic damage.
The chances of free radical formation and indirect
injury are great because cells contain 70% to 80%
water

6. OH +O H . H2O2
About two thirds of radiation-induced biologic
damage results from indirect effects

7. Biological Effects of Radiation
Biological Effects of Radiation can be broken into two groups
according to how the responses
(symptoms or effects) relate to dose (or amount of radiation received)
The First Group of biological effects are Stochastic Effects
The Second Group of biological effects are Deterministic Effects

8. Deterministic & Stochastic
Effects
Deterministic Effects are those responses which increase severity
with increased dose
Radiation injury to organisms results from killing of large no.of
cells is deteministic effect.
Sublethal damage to individual cells that results in cancer
formation / heritable mutation is stochastic effect

9. Adverse health effects of radiation exposure
Can be divided into SOMATIC EFFECTS and GENETIC EFFECTS
•Somatic effects occur in the body of the irradiated individual
•Genetic effects occur in the bodies of the offspring of the irradiated individual,
due to effects on the gonadal tissues.

12. 1. High reproductive rate (many mitoses)
2. Undifferentiated (immature)
3. High metabolic rate
Radiosensitive Cells
Cells that are more easily damaged by radiation are
radiosensitive. The characteristics of radiosensitive cells are:
Lymphocytes, germ cells, basal cells of skin and mucosa,
and erythroblasts are examples of radiosensitive cells.

13. Radioresistant Cells
1. Low reproductive rate (few mitoses)
2. Well differentiated (mature)
3. Low metabolic rate
Cells that are not as susceptible to damage from radiation are
radioresistant.The characteristics of radioresistantcells are:
Nerve and muscle cells are examples of radioresistant cells.

14. Radiation Effects at the Tissue
and Organ Level
The radiosensitivity of a tissue or organ is measured by
its response to irradiation
SHORT-TERM EFFECTS
If continuously proliferating tissues (e.g., bone marrow, oral mucous
membranes) are irradiated with a moderate dose, cells are lost
primarily by mitosis linked death.
Tissues composed of cells that rarely or never divide (e.g., muscle)
demonstrate little or no radiation induced hypoplasia over the short
term.

15. LONG-TERM EFFECTS
The long-term deterministic effects of radiation on tissues and
organs depend primarily on the extent of damage to the fine
vasculature.
Damage to capillaries leads to narrowing & obliteration of
vascular lumens .
This impairs transport of oxygen, nutrients &waste products
and result in death of all cell types dependent on this vascular
supply.

16. Radiation Effect Modifiers
The biological response to radiation is dependent
on several different factors. These include:
• Total Dose: the higher the radiation dose, the greater
the potential cellular damage.
• Dose Rate: A high dose given over a short period of
time (or all at once) will produce more damage than the
same dose received over a longer period of time.

17. Radiation Effect Modifiers (continued)
• Total Area Covered: the more cells that are exposed to
radiation, the greater the effects will be.
•Type of tissue: As discussed earlier,
radiosensitive cells are more likely to be damaged by radiation
than are radioresistant cells.
• Age: Because the cells are dividing more frequently in a
growing child, young people are affected more by radiation than
are older people.
•

19. ABSORBED DOSE
Measured in Gray (Gy) or Rad
amount of energy deposited/ absorbed per
kilogram of tissue
1Gy = 1 Joule/Kg
100 Rad = 1 Joule/Kg

20. EFFECTIVE DOSE
The effective dose is used to estimate risk in humans.
It allows the risk from exposure to one region of the body
to be compared with the risk from exposure to another
region.
It also considers the radio sensitivities of
different tissues for cancer formation or heritable effect
1Sv = 1 Joule/Kg
100 Rem = 1 Joule/Kg
.

21. Effective Dose Equivalent
Exposure or dose are not related to the amount or type of tissue
covered by the x-ray beam. A dose (or exposure) of 1 Sv could
cover just the teeth or the entire body.
Obviously, the overall effects would be different, even though
the dose is the same.
The effective dose equivalent takes into account the dose, the
volume of tissue covered and the radiosensitivity of the cells.

23. STOCHASTIC EFFECTS
CARCINOGENESIS
Radiation causes cancer by modifying the DNA.
Although most such damage is repaired, imperfect
repair may be transmitted to daughter cells and
result in cancer.
Most investigators believe that radiation acts as an
initiator, that is, it induces a change in the cell.

27. Thyroid Cancer
The incidence of thyroid carcinomas increases in
human beings after exposure.
Susceptibility to radiation-induced thyroid cancer is
greater early in childhood than at any time later in
life, and children are more susceptible than adults.
Females are 2 to 3 times more susceptible than males
to radiogenic and spontaneous thyroid cancers.

28. Salivary Gland Cancer
The incidence of salivary gland tumors is increased
in patients treated with irradiation for diseases of
the head and neck
An association between tumors of the salivary
glands and dental radiography has been shown,
the risk being highest in persons receiving full-
mouth examinations before the age of 20 years.

29. HERITABLE EFFECTS
These are changes seen in offspring of
irradiated individuals.
They are the consequences of
damage to genetic materials of
reproductive cells.

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