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Lecture 4 24-feb-2013
Radiation Biology
Today we will talk about radiation biology. What will happen to our body chemically
and biologically when taking an x-Ray image? Your reference is as usual; Dental
radiography Principles and Techniques 4th
or 3rd
edition chapter 4.
Radiation Injury
When x-rays hit our body, some chemical changes and biological changes will happen
mainly due to ionization and free radical formation.
Mechanisms of Injury
IonizationIonization results from photoelectric absorption or Compton scatter. Photoelectric
absorption occurs when x-ray gives up all its energy to an inner shell electron and
will displace the electron. Compton scatter, when x-ray interact with outer shell
electron and it will cause recoil scattering of electron and scattering the x-ray photonand we will have recoil electron. So, ionization is an atom carrying positive charge
when it loses electron and negative charge when it gets electron. These free
electrons can interact with other atoms causing chemical and biological changes.
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Free radical formationAnother way of damaging cell and usually the process is indirect. What is free radical?
Free radical is uncharged (neutral atom) that exists with a single, unpaired electron.
It is highly reactive and unstable. Free radical usually form when x-ray interactingwith water molecules in our body. Because our body is rich in water, most of
interactions of x-rays will be with water (indirect effect).
So, the radiation damage can be described in two theories; direct theory and
indirect theory.
Theories of Radiation Injury
a. Direct theoryDirect theory is when the x-ray hit the atom/molecule itself, or critical molecule
within the cell. The most sensitive one is DNA because progress in DNA can result in
death of cell. Nucleus is more sensitive than
cytoplasm and DNA is more sensitive than RNA.
b. Indirect TheoryIndirect theory is when x-ray absorbs by water
and free radical formation. Free radical will
then interact with critical molecules causing
biological damage. An indirect injury results
because the free radicals combine and formtoxins (H2O2) which cause cellular dysfunction.
The law of Bergonie and TribondeauThe law says that the most radiosensitive cell are those that have high metabolic rate,
undergo many future mitosis and are the most primitive in differentiation (actively
divide). So usually for young people or when the in uterus are sensitive to radiation.
So, depending on the cell mitotic rate, much future mitosis the more damage willhappen. Most primitive in differentiation are undifferentiated cells not mature cells.
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Mature cells are radio resistant. However this rule of Bergonie and Tribondeau are
not accepted by all the scientists over the world. Because they said with many
mitosis, more problems will be expressed/ can be seen more. So its not necessarily
that it is radiosensitive.
Dose-response curveThe correlation of damage with radiation dose. When we increase the dose
sometimes well increase the damage.
This happens and we call this as
linear (straight line) relationship. Increase
the dose will increase the damage.
Non threshold dose response is
referred to as stochastic effects (doesnt
depend on the dose). No matter how small
is the dose, this can results in damage.
In dentistry, we are mainly dealing with stochastic effect (non threshold). A
nonthreshold relationship indicates that a threshold dose level for damage does not
exist. When we have threshold, we increase the damage above the threshold, thedamage will increase. In case of non threshold/ stochastic effects, when we increase
the dose, well also increase the chance of damage. Stochastic effects do not have a
dose threshold. The effects do not depend on the absorbed dose. Examples of
stochastic effects include cancer induction and genetic mutations.
Examples of nonstochastic effects include erythema, loss of hair, cataract formation,
and decreased fertility.
REMEMBER :1) chance effects : stochastic effects : nonthreshold (not everyone will get
the disease)
2) determined dose : nonstochastic effects : threshold (when reach certain
threshold, everyone exposed will get the disease)
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Sequence of radiation injurySometimes when we give radiation, it will result in damage immediately especially
when the dose is large; this is known as acute radiation syndrome. But sometimes it
takes time for the signs and symptoms to appear especially when the dose is smalland over the time given dose.
Now we have something called latent period. A latent periodcan be defined as time
between getting the dose and seeing the observable clinical signs (manifestations).
Period of injurywill occur after latent period. The last one is recovery period.
Recovery period is when our body is able to correct problems from radiation injury
especially when the dose is small. Some says that damage actually cant be corrected
by our body and this will lead to accumulation of errors this is cumulative effects
irreversible. And this is the most dangerous period effect.
Making small x-rays in dental practice only use low dose. However its still
dangerous due to stochastic effects and also cumulative effects. Many x-rays are
exposed to our body either in low or high dose and we dont know how our body will
deal with this x-rays. Sometimes it can result in damage which cant be repaired
suddenly cancer appears.
Critical organsSome organs in our body are known as critical organs; the most effective bioradiation.
When these organs get damage, this will sacrifice humans life causing major changes
in our life. These critical organs when theyre damaged can result in cataracts, bone
marrow leukemia, salivary gland cancer, thyroid cancer and skin cancer. Critical
organs in our body will diminish humans life when it gets radiation injury.
Determining factors for radiation injuryThe factors that influence radiation injury are:-
a) Total dose: When we increase the total dose, well increase the damage.However, it depends whether it is stochastic or nonstochastic effects.
b) Dose rate: When we give large dose for short period of time this will causedamage. Unlike when we take small doses in long period of time.
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c) Amount of tissue irradiated: Total-body irradiation (large area of the body)produces more adverse effects than if small, localized areas of the body are
exposed.
d) Cell sensitivity: Highly sensitive cells such as rapidly dividing cells and youngcells that undergo mitosis will get more damage.
e) Age: Young kids, children, in uterus are more sensitive to radiation.Radiation effectsRadiation effects can be divided into short term and long term effects, somatic
and genetic effects and effects on cells, tissues and organs.
Short-Term and Long-term effects
Short-term effects - when you receive a large dose in short period of time. Acute
radiation syndrome (ARS) is a short-term effect and includes nausea, vomiting,
diarrhea and hemorrhage.
Long-term effects when small amount of radiation is absorbed repeatedly over a
long period of time. This can be related to cancer and genetic defects.
Info :Short term effects can be seen within minutes, days or weeks
whereas long term effects appears after years, decades or
generations.
Somatic and genetic effects
Somatic cells are all the cells in our body except the reproductive cells (gonads). So
what is somatic and what is a genetic effect?
Somatic effects: When the damage is expressed by the person who got the radiation.
Only the person who got the radiation is affected. Somatic effects of radiation
include cancer and cataracts.
Genetic effects: When the damage is not expressed by the person who got the
radiation but can be seen on offspring. Radiation-induced mutation affects the
health of offspring and the damage cant be repaired.
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So, somatic effects are seen in the person irradiated and genetic effects are not seen
in the person irradiated but are passed on to future generations.
Radiation effects on cells
How about the cells? The cells can be classified into highly sensitive, medium and low
sensitivity to radiation. Nerve and muscles have the least sensitivity (resistant) to
radiation. The highest sensitivity cells to radiation are lymphocyte, bone marrow,
reproductive cells.
Radiosensitive Radioresistant
Lymphocyte Mature bone
Bone marrow Muscle
Reproductive cell nerveImmature bone
Radiation effects on organs
The organs can also be classified into radiosensitive organs and radioresistant organs.
Radiosensitive organs are composed of radiosensitive cells. (Dr Mustafa asks us to
memorize all the organs/cells listed)
Radiosensitive Radioresistant
Lymph tissue Salivary gland
Bone marrow kidney
Intestine liver
Critical organs exposed during dental procedure include the following:
a) Skinb) Thyroid glandc) Lens of the eyed) Bone marrow
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We can also divide
induced cancer. E.g.: s
Radiation measu
Units of measurement
In the past we have tradi
(SI system).
Traditional units ia) Roentgen (R)b) Radiation absorbec) Roentgen equivale
For x-rays, 1R is equal to
The SI units of radia) Coulombs/kilograb) The gray (Gy)c) The sievert(Sv)
Exposure measurement
The measurement of ioni
measured by roentgen (R
as coulombs/kilogram (C
Dose measurement
The absorbed dose, the a
the tissue to the list of tissue suscepti
alivary gland.
ements
ional units but nowadays we use the i
cludes :
dose (rad)
nt (in) man (rem)
1 rad and equal to 1 rem.
tion measurement are :
(C/kg)
zation in air (amount of radiation) pro
). The new measurement equivalent t
kg).
mount of energy absorbed by tissue i
ble to Radiation
nternational system
duced by x-rays is
o roentgen is known
called as rad in
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traditional unit. Rad is radiation absorbed dose. The new SI unit is gray(Gy). The
conversion for rad and Gy can be expressed as follows :
1 rad = 0.01 Gy
1Gy = 100 rads
Dose equivalent measurement
We have something called dose equivalent and effective dose. What is dose
equivalent? And what is effective dose? We use dose equivalent (rem) when we
want to convert the effect between different types of radiation. So now we have
something called as weighing factor/quality factor. Alpha can results more damage
to our body compared to X-rays. Why? Because of dose equivalent measurement,
the weighing factor of alpha is small. We have something call linear energy transfer
(LET); the rate of energy deposition by radiation. Alpha particles, eventhough theyre
slow, they have mass so they will depart more energy, and the rate of energy
deposition in our body is higher than x-ray. Alpha is high linear energy transfer, not
fast like x-rays in general, so will deposit more energy.
The traditional unit of dose equivalent measurement is rem. The SI unit equivalent to
rem is sievert(Sv).
1 rem= 0.01sv
1 Sv = 100rems
So, any form of radiation produce the same radioeffect is known as 1 rad x-ray
radiation. So this is to calculate biologic effect of different rate of radiation.
Another question; what about the effect of radiation dose progress in our
body? Do you thinkthe effect will be the same? No! why? Some parts of our bodyare highly sensitive than other areas. So we have also weighing factor for our body.
Effective dose = biggest equivalent dose x tissue weighing factor. (Unit: Sv)
The greater risk of damage is when we have higher weighing factor. So all these can
be converted between different tissues in our body. This is called effective dose.
Radiation risks
Sources of radiation
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As you know, there are natural sources and man-made/artificial sources. Natural
sources mainly the radon gas which is present from the ground and it will come out
from ground for example and it will go to the lung. So, Radon is the main source of
natural radiation. The man-made radiation especially in the medical field, clinical
field, nuclear medicine, x-ray diagnosis and others. (All can be seen in the pie chart
below). We also have radiation from the stars or from the earth.
Dental radiation usually includes only 1% or less than 1% of the total radiation source.Cosmic radiation originates from stars and sun. Terrestrial radiation occurs naturally
and emitted from materials present in the earth and air (rocks & soil).
Risk Estimates
The potential risk of dental radiography inducing a fatal cancer in an individual is
approximately 3 in 1 million. The risk of a person developing cancer spontaneously is
much higher; 3300 in 1 million. Theyre just numbers (as the doctor said lol)
Patient exposure and dose
Amount of radiation varies. There are many factors controlling the exposure dose in
dentistry. Among them are receptor sensitivities, the technique used, kilovoltage
used and collimation.
a) Receptor sensitivity: F-speed film is the highest speed equals only 50-60% of D-speed film. Using F speed film instead of E speed film reduced additional 20%
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more of absorbed dose. Digital receptors require less radiation almost half the
dose of normal radiation on normal films.
b) Technique used: for extraoral radiation, we used the intensifying screensbecause this will increase the dose. It will change the x-ray collide and the light
will expose effect. Cone beam computed tomography need more x-ray than
intraoral. Cone beam CT is CT machine developed for dental propose. The dose
is less than medical. Cone beam CT is 3D imaging modalities.
c) Exposure factors : The use of higher kilovoltage peak reduce the skin dosed) Collimation: Exposure can be limited by using rectangular collimation. Around
60-70% absorbed dose can be reduced.
ALARAWhat is ALARA? ALARA is As Low As Reasonably Achievable. What does this
mean? This means that you must try your best to decrease the dose as much as
possible. Dental radiograph should be prescribed only when the disease
progression outweights the risk of the damage. So you have to ask yourselves, Will
I make benefits from this x-ray?, if theres no benefits why should I do this x-ray?!
For example occlusal caries, most of the time occlusal caries cant really be seen on
radiograph. So why do u ask patient to take x-ray while you can detect this lesion
clinically? This is not allowed in dental practice, this is not ALARA.Another example, why you ask patient to take cone beam CT to detect endodontic
lesion while you can see it in periapical radiograph? I know clinically that this pulp is
not vital but of course I need radiograph to detect the working length for the dental
treatment. Theres no need to ask the patient to take cone beam CT when
radiograph is enough unless you can get benefits from it. Another one example; TMJ
displacement will not get any benefits neither from cone beam nor from radiograph.Major salivary glands irradiation
When our salivary gland receives radiation, this will result in more viscous secretion.
Parotid gland is the most radiosensitive salivary gland. The effect will depend on the
dose given.
Pediatric radiation
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If radiation precedes the calcification it may destroy the tooth part. However the
eruption is radioresistant. Radiation cause shortening of root formation, abortion
root formation but the teeth still erupt even with abnormal root. It is dose
dependent, like in the case of salivary glands. And patients who had radiation
therapy should avoid taking intraoral radiograph during six months after completing
the treatment, so you should postpone taking any radiograph for such patients for 6
months.
Osteo-radio-necrosis
Necrotic bone: dead bone. Radionecrosis means radiation will lead to necrosis of
bone. This radiation causes damage mainly to the periosteum and the cortical bone.
After radiation, this can lead to necrosis especially when there is infection or
trauma or mechanical damages. There should be another reason other than
radiation to cause the damage. Mandible is more affected than maxilla because of
less vascularity in the mandible. The higher the dose is the higher the risk of getting
radionecrosis (as seen in stochastic effect).
The male was more radiosensitive than the female. The males carried most ofthe radiation induced genetic burden.
The amount of radiation required to double the natural and spontaneousmutation rate is between 20 to 200 rads.
THE END
p/s : Sorry for the super late submission of this script T_T forgive me for any mistakeswritten.
Done by: Aina Amalia
Corrected by Sawsan Jwaied
Thanx Aina for your great work