RADIOBIOLOGY Speaker Sharib Ahmed on 19-11-2013 - Copy

8/12/2019 RADIOBIOLOGY Speaker Sharib Ahmed on 19-11-2013 - Copy

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BASIC RADIOBIOLOGYSHARIB AHMEDTrainee Medical Physicist

Trainees Intellectual ForumACADEMIC SESSION


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Introduction

Ionizing radiation Linear energy transfer

Direct and indirect action of radiation

Biological effects of radiation Radiation dose and units

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Radiobiology Radiobiology is the study of the action of ionizing radiations on living

things.

The absorption of energy from radiation in biologic material may lead to

excitation or to ionization.

The raising of an electron in an atom or molecule to a higher energy level

without actual ejection of the electron is called excitation.

If the radiation has sufficient energy to eject one or more orbital electrons

from the atom or molecule, the process is called ionization

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The first Radiobiology experiment

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IonizationIonization is the process of removing one or more electrons from atoms by the

incident radiation leaving behind electrically charged particles (an electron and

a positively charged ion) which may subsequently produce significant

biological effects in the irradiated material.

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Ionizing radiation The important characteristic of ionizing radiation is the localized release of

large amounts of energy.

The energy dissipated per ionizing event is about 33 eV, which is more

than enough to break a strong chemical bond; for example, the energy

associated with a C=C bond is 4.9 eV.

For convenience it is usual to classify ionizing radiations aselectromagnetic or particulate.

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X- and y-rays X- and y-rays do not differ in nature or in properties; the designations x- or

y-rays reflects the way in which they are produced.

X-rays are produced extranuclearly.y-rays are produced intranuclearly.

In practical terms this means that x-rays are produced in an electrical

device that accelerates electrons to high energy and then stops them

abruptly in a target, usually made of tungsten or gold. Part of the kineticenergy (the energy of motion) of the electrons is converted into x-rays.

On the other hand, y-rays are emitted by radioactive isotopes; they

represent excess energy that is given off as the unstable nucleus breaks up

and decays in its efforts to reach a stable form.

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Charged elementary particles

The passage of charged particles, electrons and positively charged ions,

causes intense damage (energy deposition) to molecules along the path inliving tissue due to strong electrostatic interactions between the travelling

particle and the electrons of the atoms of the medium.

Protons with one unit mass and one positive charge, cause less damage

than particles (helium nuclei) because the rate of deposition of energyvaries inversely in proportion to the velocity of the particle and directly in

proportion to the square of the charge.

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Charged elementary particles

particles: They are a highly ionizing form of particulate radiationthey usually have low penetration. They quickly lose their energy and theypenetrate only a few tens of microns in body tissue.

Beta particles (, electrons):Beta particles (, electrons) are also

emitted by radioactive nuclei they carry a single negative charge but theirpath in absorbing materials such as tissue is erratic due to their light mass.Generally, beta particles do not penetrate further than the skin of the human

body.

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Uncharged particles

Neutrons:

Uncharged particles with a mass very similar to that of a proton and are anindirectly ionizing radiation. Neutrons interact with the atomic nuclei of the

medium and they lose energy by different interaction processes depending

on their energy (velocity) and the mass of the encountered nucleus.

In soft tissues, because of the abundance of protons with mass equal to thatof neutrons, fast neutrons (>1 MeV) mostly lose energy by elastic

scattering through collision processes producing high energy recoil

protons, which in turn deposit energy by electrostatic interactions with

electrons in the tissue as described above.

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Ions The nuclei of carbon, neon, silicon, argon atoms form charged ions when

one or more orbital electrons have been stripped off.

Ions can be accelerated to hundreds of MeV energies in special accelerator

facilities. High energy charged ions offer special advantages in cancer

radiotherapy because of the energy distribution along their track which has

a high peak at its end (the Bragg peak).

This allows the possibility of depositing high energy densities at depth in

tissue but these facilities are as yet very limited on account of high costs

and sophisticated technical requirements.

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Linear energy transfer When ionizing radiations traverse through matter, they lose energy

gradually through various interaction processes along the length of their

path. For a particular absorber, the rate of loss of energy depends on the

energy and type of radiation as well as the density of the material.

The density of energy deposition in a material such as tissue is called the

Linear Energy Transfer (LET) of the radiation.

It is defined as the average energy deposited per unit length of track of

radiation and the unit is keV/ m.

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Linear energy transfer LET essentially indicates the quality of different types of radiation and is

important because the biological effect of a radiation depends on its

average LET.

Charged particles generally have higher LET than X and rays because of

their greater energy deposition along the track.

In general the relative biological effectiveness (RBE) of a radiationincreases with its LET up to a value of about 100 keV/m and above this

value starts to decline due to energy deposition in excess of that needed to

cause the biological effect (overkill).

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TYPICAL LET VALUES OFIONIZING RADIATION

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ABSORPTION OF X-RAYS Radiation may be classified as directly or indirectly ionizing.

All of the charged particles previously discussed are directly ionizing

Electromagnetic radiations (x- and y-rays) are indirectly ionizing.

The process by which x-ray photons are absorbed depends on the energy of

the photons concerned and the chemical composition of the absorbingmaterial.

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DIRECT AND INDIRECT ACTION OFRADIATION The biologic effects of radiation result principally from damage to DNA,

which is the critical target.

If any form of radiationx- or y-rays, charged or uncharged particlesis

absorbed in biologic material. The atoms of the target itself may be ionized

or excited, thus initiating the chain of events that leads to a biologic

change. This is called direct action of radiation.

Alternatively, the radiation may interact with other atoms or molecules in

the cell (particularly water) to produce free radicals that are able to diffuse

far enough to reach and damage the critical targets. This is called indirect

action of radiation.

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DIRECT AND INDIRECT ACTION OFRADIATION

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INDIRECT ACTION

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Biological effects of radiation

Biological effects of radiation arise when ionizing radiation interacts with

an organism/tissue and leaves some energy behind. The process by which

electromagnetic photons are absorbed in matter depends on their energy

and the atomic number of the absorbing material.

Photons passing through matter transfer their energy through the following

three main processes: photoelectric absorption, Compton scattering, and

pair production.

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Photoelectric absorption In photoelectric absorption, the photon interacts with a bound inner shell

electron in the atom of the absorbing medium and transfers its entire

energy to the electron ejecting it from the occupied atomic shell.

The photoelectric effect is the dominant energy transfer mechanism for X

and ray photons having energies below 50 keV in biological tissues, but it

is much less important at higher energies.

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Compton scattering The incident photon interacts with the outer orbital electron whose binding

energy is very low compared with that of the incident photon. In thisinteraction, the incident photon transfers energy to an atomic electron causingits ejection from the atom.

The photon is scattered with the remainder of the original energy in a differentdirection to that of the incident photon. Compton scatter thus causes ionizationof the absorbing atom due to loss of an electron.

The probability of compton scattering

Decreases with increasing photon energy.It is the principal absorption mechanism forX and rays in the intermediate energy rangeOf 100 kev to 10 mev.

This range is in the therapeutic radiation range

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Pair production

When a photon of high energy ( >1.02 MeV) interacts with atoms of the

medium, the incident photon can be spontaneously converted into the mass

of an electron and positron pair by interaction of the Coulomb force in the

vicinity of the nucleus.

The oppositedly charged particles are emitted in opposite directions to each

other and cause damage as secondary charge particles.

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Dependence of absorption on atomicnumber

Compton process is nearly independent of atomic number. Compton and

photoelectric effects are vital for appropriate applications in X- ray

diagnosis and cancer therapy.

In radiotherapy, high-energy photons in the range of 1-10 MeV are

preferred because absorbed dose is nearly the same in bone and soft issues

whereas low energy photons are preferred in diagnosis because of the

much desired large contrast in absorption of these tissues.

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Radiation dose and units

The biochemical changes produced by ionizing radiations are the

fundamental events leading to radiation damage in tissues. Radiation is

measured either as exposure or as absorbed dose. The absorbed dose is

the amount of energy absorbed in a system.

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Radiation dose and unitsAbsorbed dose:

The amount of energy absorbed per mass is known as radiation dose.

Radiation dose is the energy (Joules) absorbed per unit mass of tissue and

has the (S.I.) units of gray (1 Gy = 1 J/kg).

In the past the rad (radiation absorbed dose) was used, where

100 rad = 1 Gy (1 rad = 1cGy).

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Radiation dose and unitsEquivalent dose:

As discussed above the biological effectiveness (RBE) of each type ofradiation varies greatly depending largely on LET. For radiation protection

and occupational exposure purposes the term equivalent dose is used to

compare the biological effectiveness of different types of radiation to

tissues.

The (S.I.) dose equivalent (HT) in Sievert (Sv) is the product of the

absorbed dose (DT) in the tissue multiplied by a radiation weighting factor

(WR) often called the QUALITY FACTOR.[(HT) = (WR)x (DT)]

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Effective dose

Effective Dose is used to estimate the risk of radiation in humans. It is sum

of the products of equivalent doses to each organ/tissue (HT) and thetissue weighting factor (WT).

The unit of effective dose is the Sievert (Sv).

E = (WT) x (HT)

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Collective dose

Collective dose is defined as the dose received per person in Sv multiplied by

the number of persons exposed per year i.e. man-sievert per year. This unit is

generally used for protection purposes and in population response calculations.

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References

I. RADIATION BIOLOGY: A HANDBOOK FORTEACHERS AND STUDENTS IAEA

II.Radiobiology for the RadiologistEric J. Hall D.PHIL. D.SCF.A.C.R. F.R.C.R.

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RADIOBIOLOGY Speaker Sharib Ahmed on 19-11-2013 - Copy

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