DR. MOHAMMED MOSTAFA EMAM

INAYA MEDICAL COLLEGE

(IMC)RAD 232 - LECTURE 3, 4 & 5

Biological Effects of Ionizing Radiation & Commonly Used Radiation Units

How does radiation injure people?

• High energy radiation breaks chemical

bonds or DNA molecules.

• This creates free radicals, like those produced

by other insults as well as by normal cellular

processes in the body.

• The free radicals can change chemicals in the body.

• These changes can disrupt cell function and may kill cells.

+

-

Genomic

Instability

Sometimes DNA

damage produces

later changes

which may

contribute to

cancer.

Gene

Expression

A gene may

respond to the

radiation by

changing its

signal to

produce

protein. This

may be

protective or

damaging.

Effects of DNA Damage

DNA is the most important moleculethat can be changed by radiation

Studies have shown that most radiation-induced DNA

damage is normally repaired by the body

Gene Mutation

Sometimes a

specific gene is

changed so that it

is unable to make

its corresponding

protein properly

Chromosome

AberrationsSometimes the

damage effects the entire

chromosome, causing it to break or recombine in an

abnormal way. Sometimes parts of

two different chromosomes may

be combined

Cell Killing

Damaged DNA

may trigger

apoptosis, or

programmed

cell death. If

only a few cells

are affected,

this prevents

reproduction of

damaged DNA

and protects

the tissue.

How does this damage from ionizing

radiation effect our bodies?

Cancer

Sufficient Cell Killing Sufficient Genetic

Alterations

Radiation Sickness

RadiationDose

Radiation Dose

One of the most confusing things about

understanding radiation effects is visualizing “how

much” radiation is involved. It is very difficult to keep

the units which measure radiation straight. A number

describing the amount of radiation means nothing

without evaluating the units, but this is not easy.

For example...

...try to match the letter with the amount of radiation involved in each example

• Amount of potassium 40 in the body

• Dose to Atomic bomb survivors

• You can safety hold this amount of alpha radiation

• One coast to coast flight

• A diagnostic X-ray

A. Billions of becquerels

B. About 250 picocuries

C. 2-10 millirem

E. 2

millirads

D. 0-5 Gy

Commonly Used Radiation UnitsAbsorbed dose (Gray or rad)

Average dose

Organ dose

Dose commitment

Collective dose

Effective dose (Sievert or rem)

Committed effective dose

Equivalent dose

Collective equivalent dose

Committed equivalent dose

Uniform equivalent dose

Dose equivalent

Collective dose equivalent

Ambient dose equivalent

Directional dose equivalent

Individual dose equivalent

Individual dose equivalent, penetrating

Individual dose equivalent, superficial

Dose and dose-rate effectiveness factor

Man-gray

Man-sievert

Tissue weighting factor

Relative biological effectiveness (RBE)

Quality factor (Q)

Fatality probability coefficient

Nominal fatality probability coefficient

Radiation weighting factor (wR)

Linear energy transfer (LET)

Radioactivity (Becquerel or curie)

Each of these units has a

different technical meaning. All

are used by experts to talk

about radiation. With so many

terms, you can see why it is

important to know what the unit

means when you are evaluating

radiation information.

RAD

BecquerelSievert

ActivityThe number of times, each second, a radioactive material decays and releases radiation.

Exposure Amount of ionization per mass of air due to x and gamma rays.

Dose (Absorbed)The amount of radiation energy absorbed into a given mass of tissue.

Dose (Equivalent) H & Effective dose equivalent (HE )Measures the energy per unit mass times adjustments for the type of radiation;

Involved Radiation (quality factor) and the biological response in the tissue (a weighting factor).

* Equivalent dose converts dose into a measure of risk.

Understanding Radiation Units

Activity• Disintegration/sec=1 Becquerel (Bq)

• 37 billion Bq = 1 curie

Exposure

• Roentgen

Dose (Absorbed)• 1 joule/kg=1 Gray(Gy)

• 1Gray=100 rad =100,000 mrad

Dose (Equivalent)• Gray x quality factors= Sievert (Sv)

• 1 Sievert =100 rem =100,000 mrem

Understanding Radiation Units

Standard Units

S.I. Units

What is the meaning of activity?

This is the expectation rate of spontaneous nuclear transitions in a source.

Becquerel = 1 disintegration/second.

• This is the SI unit for measuring radioactivity.

Curie.

• Defined as 3.7 x 1010 disintegrations per second= 3.7 x 1010 Bq.

This is the activity of 1 gram of radium

in equilibrium with its decay products

• The Rutherford. 1 Rd = 106Bq.

ACTIVITY

How much is a Becquerel (Bq)?

• The natural 40K activity in the body of an adult human of normal weight is4000-6000 Bq.

• There is an average of about 50 Bq per cubic meter of air inside a homefrom radon.

• Even though a 60Co source of strong gamma radiation containing billions ofBq can kill you if you are standing 5 meters from it, it is harmless at adistance of 100 meters.

• A Bq has 27 times more disintegrations than a pCi, but is still a very smallamount of radiation.

ACTIVITY

How much is a picocurie (pCi)?

•Many times the media reports excess radiation in picocuries. It takes 1,000,000,000,000 pCi to make 1 Curie.

A Becquerel is 1 disintegration/second. It takes 27 pCi to make one Bq, so a pCi represents less radioactivity that a Bq and results in very, very little dose.

ACTIVITY

What is the meaning of exposure?

• The quantity of X- or gamma-radiation to which an object is exposed.

• This electromagnetic radiation produces ionization within the object.

Amount of ionization per mass of air due to x and gamma rays. This is the amount of ionization produced by photons in air.

• Since it is impossible to directly measure the absorbed dose in tissue, the measurement of radiation is performed in air.

• It is measured in roentgen (R) and Sieverts (Sv).

Exposure

What is the meaning of exposure?

• Roentgen.

• Röntgen or Roentgen may refer to:

• Roentgen (unit), unit of measurement for ionizing radiation, named after Wilhelm Röntgen

• Wilhelm Röntgen (1845–1923), German physicist, discoverer of X-rays

Exposure

What is the meaning of exposure?

• Roentgen.

• This is defined as the amount of gamma radiation that produces 1 cm3 of air ionization equal to 1 electrostatic unit (esu).

• 1 esu = 3.3 x 10-10 coulombs = 2 x 109 ion pairs/cm3 of air.

• Equivalent to 2.58 x 10-4 C/kg air (0.0087 J/kg of air).

• 1R is approximately 10-2 Sv.

Exposure

ROENTGEN

• Roentgen was defined as 1R=1 electrostatic unit (esu)/cm3 airat standard temp and pressure(STP) = Δ Q by Δ m;

Where:

Δ Q is the absolute value of total charge of ions of one sign produced inair when all the electrons liberated by photons in air of mass (Δ m).

X = Δ Q/ Δ m

Conventional units is Roentgen

SI unit : c/kg

1R=2.58*10-4 c/kg

What is the meaning of absorbed dose?

• This is the energy imparted/ given to matter by charged or uncharged ionizing particles.

ABSORBED DOSE

What is the meaning of absorbed dose?

Gray

1Gy = 1 J/kg.

This is the SI unit for absorbed dose of ionizing radiation.

The Rad. 1 rad = 10-2Gy (= 10-2 J/kg).

• This is defined as the amount of radiation that deposits 100 ergs (10-5 J) in each gram of tissue it traverses.

• Two different types of radiation may, however, produce different degrees of biological damage even though they are both rated as 1 rad.

ABSORBED DOSE

How much radiation is an X-ray?ABSORBED DOSE

• 1Gray=100 rad =100,000 mrad

• So, the average chest X-ray may give a dose;

10 millirads = 0.01 rads = 0.0001 Gray.

• A millirad is comparatively small. Average normal background

level of radiation is 370 mrad/year.

• One Gray is a relatively large amount of radiation. If 3-4 Gray

are delivered over a short time to the whole body, they can be

deadly.

What is the meaning of equivalent dose?EQUIVALENT DOSE

This is the quantity used to express on a common scale the risk to exposed persons from all ionizing radiations.

Biological effects of ionizing radiation 22

Dose Equivalent

Since different radiations have different harmful effects on

human tissues.

It is measured in Sieverts (Sv).

H = D×Q.F.

H = equivalent dose (Sv)

D = dose (Gy)

Q.F. = radiation quality factor of radiation (no unit)

1 Sv = 1 J/kg = 100 rem

What is the meaning of equivalent dose?EQUIVALENT DOSE

Sievert

1 Sv = 1 J/kg. The Sievert is equal to the absorbed dose in tissue (Gy) multiplied by the 'quality factor' for the particular type of ionizing radiation. The quality factor is a dimensionless number representing the relative effect produced by the same absorbed doses of different types of radiation. In older terminology the quality factor was referred to as the Relative Biological Effectiveness (next slides)

rem (roentgen equivalent man): 1 rem = 10-2Sv (= 10-2 J/kg).

This is defined as the amount of radiation which when absorbed by a person, will produce the same biological effects as the absorption of 1 roentgen of x-ray or gamma-ray radiation.

RADIATION Q.F.

X-rays & gamma rays 1.0

Electron (incld. β-rays) of energy >30kv

1.0

Thermal ( slow) neutron 5

Fast neutrons 20

Conventional unit is Roentgen equivalent in man(rem)

SI unit is Sivert

1rem=10 Sivert

Dose Equivalent

Radiation Q. Factors Illustration

Type and Energy Range Q. Factor

X and γ rays, electrons, positrons and muons 1 Neutrons <10 keV 5 Neutrons 10 keV to 100 keV 10 Neutrons >10 keV to 2 MeV 20 Neutrons > 2 MeV to 20 MeV 10 Neutrons >20 MeV 5 Protons, other than recoil protons and energy

>2 MeV 2

Alpha particles, fission fragments, nonrelativistic

heavy nuclei 20

EQUIVALENT DOSE

What is Radiation Quality Factor?EQUIVALENT DOSE

Different types of radiation behave in different ways. In order to compare the amount of risk or biological change that occurs, quality factors are introduced.

Biologic effects of radiation depend not only on dose but also on the type of radiation.

What is Radiation Quality factor?EQUIVALENT DOSE

For example:

• The damage produced by 1 Gy of x-radiation is equal to that produced by 1 Gy of gamma radiation. Thus, gamma radiation has a quality factor of 1 or 1 Gygramma rays x 1 =1 Sv.

• The damage produced by 20 Gy of x-radiation is equal to that from 1 Gy of alpha radiation. Alpha radiation has a quality factor of 20 or 1 Gy of alpha radiation x 20 = 20 Sv.

• Quality factors for other types of radiation are between 1 & 20.

What is the meaning of equivalent dose?EQUIVALENT DOSE

To define the rem quantitively, a relative biological effectiveness (RBE) has been established

(number of rem) = (number of rad) x RBE. The following table gives RBE for the usual types of radiation.

RELATIVE BIOLOGICAL EFFECTIVENESS

Type of radiation rad x RBE = rem

x-rays and gamma-rays 1 1 1

Beta radiation 1 1 1

Protons 0.1 10 1

Alpha particles 0.05 20 1

Fast Neutrons 0.1 10 1

Slow Neutrons 0.3 3 1

Radiation induced cancers have been seen in the atomic bomb survivors exposed to as low as 0.2 Sieverts.

A Sievert is a relatively large amount of radiation.

The annual background radiation exposure for a typical American is 0.0037 Sv, 3.7 mSv or 370 millirem.

1 Sv = 100 rem1000 mSv = 100,000 mrem

EQUIVAENT DOSE

How much is a Sievert (Sv)?

EQUIVAENT DOSE

How much is a millirem (mrem)?

• The annual background radiation exposure for a typical

American 370 mrems.

• The average dose from watching color TV is 2 mrem each year.

• The granite from Grand Central Station exposes its employees

to 120 mrem of radiation each year

• People in Denver receive 50 mrem more each year than those

in LA because of the altitude.

• The nuclear industry contributes to less than 1 mrem/year to an

individual’s background radiation.

• A millimrem is a small unit of measure.

EQUIVALENT DOSE EFFECTIVE (HE)

• Whole body exposures are rarely uniform.

• Tissues vary in sensitivity to radiation induced effects

• Effective dose is a measure of radiation and organ system specific damage in man

• The effective dose equivalent HE =Sum of Ht x Wt

• Ht = mean dose equivalent received by the tissue t• Wt =weighing factor of tissue t

Tissue Weighting Factors Illustration

0.01 0.05 0.12 0.20

Bone surface Bladder Bone Marrow Gonads

Skin Breast Colon

Liver Lung

Esophagus Stomach

Thyroid

Remainder

EQUIVALENT DOSE EFFECTIVE (HE)

The effectiveness of the dose is dependent on the dose-rate

Dose

1 bottle of Aspirin

or

250,000 mrem of

Radiation

Dose -Rate

Over 50 seconds??

Or over 50 years??

Over 50 seconds??

Or over 50 years??

Dose-rate

Death

Minimal health risk

Death

Minimal health risk