Presented By: Dr. VandanaJunior Resident, Dept. of RadiotherapyCSMMU, Lucknow

Moderator: Mr. Teerthraj

The term radiation applies to the emission and propagation of energy through space or a material.

no mass or physical form travel at speed of light (c) in a vacuum

(or air) c = 3 x 108 m/s

travel in a linear path (until interaction occurs)

unaffected by electric or magnetic fields gravity

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obeys the wave equation

c =

In Passing through the matter, the intensity is reduced (attenuation), because of absorption & scattering.

obeys the inverse square law

I1d12 = I2d2

2

Radiation intensity is inversely proportional to the square of the distance from

the source at any place.

4

dual nature: wave vs. particle

Wave: continuously changing force fields energy travels as sine

WAVE macroscopic level

Particle: photon or quanta small packet of energy

acting as a PARTICLE microscopic level

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Ionizingionizes [strips

electrons from] atoms

Ionizingionizes [strips

electrons from] atoms

Non-Ionizingmany other modes of

interaction

Non-Ionizingmany other modes of

interaction

Electro Magnetic Spectrum (EMR)

Radiation that has enough energy to move atoms to vibrate, but not enough energy to remove electrons.

The process by which a neutral atom acquires a positive or a negative charge is known as Ionization.

Removal of an orbital electron leaves the atom positively charged, resulting in an ion pair.

• molecule with a net positive charge

• free electron with a negative charge

Non-Ionizing Vs Ionizing Radiation

Atom - The smallest indivisible part of an element.

Nuclei + Orbital e- = Atom

Nucleus protons and neutrons

Atoms are specified as ZXA where Z = atomic number, and A = mass number.

Fig : Bohr’s model of the atom

Fig : Energy level diagram (Hydrogen Nucleus)

According to Niels Bohr, electrons revolve in specific orbits around the nucleus. These orbits are named as K,L,M etc; K being innermost orbit.

These electron orbits are synonymous with energy levels.

Higher the atomic number, greater is this binding energy.

Amount of Energy required to remove an electron completely from an atom

B.E. α Z (Atomic Number )the greater Atomic Number, the greater binding energies

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When an x-ray or γ ray beam passes through a medium, interactions occur between the beam and the matter.

Initially the electrons are ejected from the atoms of the absorbing medium which in turn, transfer their energy by producing ionization and excitation of the atoms along their path.

If the absorbing medium consists of body tissues, sufficient energy may be deposited within the cells, destroying their reproductive capacity.

However, most of the absorbed energy is converted into heat, producing no biologic effect.

Process Definition

Attenuation Removal of radiation from the beam by the matter. Attenuation may occur due to scattering and absorption

Absorption The taking up of the energy from the beam by the irradiated material. It is absorbed energy, which is important in producing the radiobiological effects in material or soft tissues.

Scattering refers to a change in the direction of the photons and its contributes to both attenuation and absorption

Transmission Any photon, which does not suffer the above processes is transmitted.

High Speed Electrons

Photon

When mono-energetic (mono-chromatic) radiation passes through any material, a reduction in the intensity of the beam occurs, This is known as attenuation.

Attenuation occurs exponentially, i.e. a given fraction of the photons is removed for a given thickness of the attenuating material. Fig : Semilog plot showing exponential

attenuation of a monoenergetic photon beam.

• Half-value-layer (HVL)- The thickness of the absorber material required to decrease (attenuate) the intensity of a monoenergetic photon-beam to half its original value.

• This shows the quality or the penetrating power of an x-ray beam.

2nd HVL

1st HVL

Linear attenuation coefficient (μ) : The fractional reduction (in any monoenergetic photon-beam) for any given material per unit thickness.

μ : is the probability of the photon being removed by a given material.

μ = 0.693 / HVL

The linear attenuation coefficient depends upon the density of the material. As compression of a layer of material to one half of the thickness will not affect its attenuation.

To circumvent this problem, the mass attenuation coefficient is used which is defined as:

Mass attenuation coefficient = μ / ρ

Attenuation of a photon beam by an absorbing material is caused by five major types of interactions :

• Elastic scattering, Thomson scattering, unmodified scattering, classical scattering, Rayleigh scattering, etc.

• X-rays cause the bound electrons to vibrate. These in turn emit radiation of the same frequency in all directions.

• Wave nature of radiation

•

Attenuation without absorption- The energy is scattered in all direction, but none of the energy is absorbed.

• Little importance in practical radiotherapy, but is important in X-ray crystallography.

Inelastic scattering, Modified, incoherent.

An incident photon interacts with an orbital electron to produce a recoil electron and a scattered photon of energy less than the incident photon.

-

--

Incoming photonCollides with electron

--

--

Electron is ejected from atom

-

Scattered Photon

Before interaction

After interaction

The photon collides with electron and hands over part of its energy to it. The angle through which the photon is scattered, the energy handed over to the electron, and energy lost by the photon are interconnected.

If the angle by which the electron is scattered is Φ and the angle by which the photon is scattered is θ, then the following formula describes the

change in the wavelength (δλ)of the photon:

λ2 – λ1 = δλ = 0.024 ( 1- cos θ) Å

• The Compton effect results in both attenuation and absorption.

• The attenuation caused here is dependent upon the Electron density and is practically same for all substances except hydrogenous material, like water and soft tissue, where the Compton effect is greater (because of the higher electron density).

• It does not depend on Atomic Number.

• It is measured as mass scattering coefficient (σ/ρ),

Material Density (g/cm3) Atomic Number Number of Electrons per Gram

Hydrogen 0.0000899 1 6.00 × 1023

Carbon 2.25 6 3.01 × 1023

Oxygen 0.001429 8 3.01 × 1023

Aluminum 2.7 13 2.90 × 1023

Copper 8.9 29 2.75 × 1023

Lead 11.3 82 2.38 × 1023

Effective Atomic Number

Fat 0.916 5.92 3.48 × 1023

Muscle 1.00 7.42 3.36 × 1023

Water 1.00 7.42 3.34 × 1023

Air 0.001293 7.64 3.01 × 1023

Bone 1.85 13.8 3.00 × 1023

Data from Johns HE, Cunningham JR. The physics of radiology. 3rd ed. Springfield, IL: Charles C Thomas, 1969.

The photoelectric effect is a phenomenon in which a photon interacts with an atom and ejects one of the orbital electrons from the atom.

The photon transfers all its energy to the atom. This is used to overcome the binding energy as well as to provide the kinetic energy to the photo-electron.

hν - W + ½ mν2

W = The binding energy of the electron and ½ mν2 is the kinetic energy of the photo electron.

The ionized atom regains electrical neutrality by rearrangement of the other orbital electrons. The electrons that undergo these rearrangements surrender some of the energy in form of a photon known as the characteristic radiation of the atom.

Absorption of these characteristic radiation internally in the atom may result in emission of Auger electrons. These electrons are monoenergetic in nature.

Fig. : The photo electric effect

The mass photoelectric attenuation coefficient (τ/ρ) is directly proportional to the cube of the atomic number and inversely proportional to the cube of the radiation energy.

τ/ρ = k Z3/ E3

• As the graph on the right shows, there are discontinuities in the attenuation coefficient at specific photon energies.

• The absorption edges, correspond to the binding energies of the electrons in different shells.

• In diagnostic radiology, the primary mode of interaction is photoelectric. It is also responsible for the contrast effect.

• In therapeutic radiology, low-energy beams in orthovoltage irradiation caused excessive absorption of energy in bone.

Pair Production: When the photon with energy in excess of 1.02 MeV passes close to the nucleus of an atom, the photon disappears, and a positron and an electron appear.

Annihilation: These two particles collide, converting to 2 photons with equal energy of 511 kev.

Thus, the energy absorbed from the beam (with incident energy, E) is given by:

Eabsorbed = E - 1.02 MeV

Pair production results from an interaction with the electromagnetic field of the nucleus and as such the probability of this process increases rapidly with the atomic number (Z2).

In addition, the likelihood of this interaction increases as the photon energy increases.

The pair production coefficient (π) is directly proportional to Z2 and log of incident photon energy.

π = k Z2 log (E)

This reaction occurs when the photon has energy greater than the binding energy of the nucleus itself. In this case, it enters the nucleus and ejects a particle from it. The photon disappears altogether, and any energy possesses in excess of that needed to remove the particle becomes the kinetic energy of escape of that particle.

In most cases, this process results in the emission of neutrons by the nuclei.

This has a threshold of 10.86 MeV.

Now a days, the main use of this reaction is for energy calibration of machines producing high energy photons. For this the following reaction is used:

29Cu63 +γ 29Cu62 + 0n1

The Total Mass attenuation coefficient is the sum of three individual coefficients; photoelectric coefficient, mass scattering coefficient and pair production coefficient:

(μ/ρ) = (τ/ρ)+(σ/ρ)+(π/ρ)

At low energies the photo electric attenuation coefficient is larger.

In between the ranges of 200 KeV- 4 MeV, Compton scattering is the predominant mode of interaction.

In the ranges above, pair production is dominant.

Photon Energy (MeV)

Relative Number of Interactions (%)

P.E. (τ/ρ) Compton (σ/ρ) Pair Prod. (π/ρ) 

0.01 95 5 0

0.026 50 50 0

0.060 7 93 0

0.150 0 100 0

4.00 0 94 6

10.00 0 77 23

24.00 0 50 50

100.00 0 16 84

Data from Johns HE, Cunningham JR. The physics of radiology. 3rd ed. Springfield, IL: Charles C Thomas, 1969.

Figure: Plot of total mass attenuation coefficient (μ/ρ) as a function of photon energy for lead and water. (from Johns HE, Cunningham JR. The physics of radiology, 3rd ed.)

Energy Range

Dominant Effects

Up to 50KeV PE (Photo Electric) effect is important

60 KeV - 90 KeV

Both PE & Compton effect

200 KeV - 4 MeV

Compton effect

Beyond 20 MeV

Pair Production

Radiation type Direction

Recoil electrons Travels forward, angle not more than 90°.

Photoelectrons and electron pairs

Travels forward

Characteristic and annihilation radiation

Isotropic i.e. equally in all directions

Coherent scattered photons Isotropic

Compton scatter photons In forward direction, small angle of scattering, lesser scattering for greater incident energy

Most of electrons set in motion by the above interactions lose energy by inelastic collisions with the atomic electrons of the material.

Some electrons also loose energy by Bremsstrahlung interactions with the nuclei.

Beta Particle

-Bremsstrahlung Photon

+ +

Nucleus

Thus, the energy absorption coefficient(μen) is defined as the product of the energy transfer coefficient(μtr) and (1-g) where g is the fraction of energy of secondarily charged particles lost to bremsstrahlung in the material.

μen = μtr (1-g)

In most interactions involving the soft tissues, the bremsstrahlung component is negligible , and the energy absorption coefficient is equal to the energy transfer coefficient under these conditions.

The relationship between the mass attenuation coefficients and the mass absorption coefficient varies as per the radiation energy as follows:

Photon energy

Mass

coeffi

cient

100 KeV 1 MeV 10 MeV

91%

15%

46% 7

1%

96%

10 KeV

% of attenuated energy absorbed

μen

μ/ρ

The mass absorption coefficients are practically identical for most biological materials .

In this energy range, the absorption per gram is maximum for hydrogen, because of its higher electron density.

However in very high and very low energy ranges the high atomic number materials e.g. Bone absorb more radiation with several unfortunate consequences.

The energy absorption coefficient is an important quantity in radiotherapy since it allows the evaluation of energy absorbed in the tissues, a quantity of interest in predicting the biologic effects of radiation.

Absorption (contd.)

Particulate radiation can be classified into two categories:

◦ Ionizing or charged particles - Electron, Proton◦ Uncharged particles.

The two different modes of interaction and energy transfer of electrons with matter include :

◦ Collision between the particle and the electron cloud resulting in ionization and excitation. This is called Collisional loss.

◦ Collision between the nucleus and the particle resulting in bremsstrahlung radiation. This is called Radiative loss.

Electrons are light particles with negligible mass and single negative charge. As a result they penetrate deeper than other charged particles but at the same time undergo greater scattering.

The ionization pattern produced by a beam of electrons is characterized by a constant value from the surface to a depth equal to about half the range, followed by a rapid falling off to almost zero at a depth equal to the range.

This is specially seen in electrons in the energy range of 6 -15 MeV – making these useful in clinical practice

These characteristics make electrons a useful treatment modality for superficial lesions.

Neutrons are indirectly ionizing uncharged radiations, which interact only with the nucleus in two ways: ◦ By recoiling protons from hydrogen and the nucleus in other elements.◦ Nuclear disintegration, which contribute to ~30% of the total dose in

tissues.

The most efficient recoil is seen in the hydrogen nucleus and this leads to the maximum absorption. This is an advantage because most of the soft tissues in the body contains a large proportion of hydrogen.

The recoil protons, set in motion after interaction with neutrons. further cause ionization. The dense ionization produced by these particles in the vicinity, results in high LET values

LET has certain important radiobiological implications:

◦ High LET radiation is more likely to induce lethal damage in the cells due to the dense ionization they produce.

◦ The oxygen enhancement ratio nears 1 as the LET increases – advantage in hypoxic tumors.

◦ The effect of fractionation reduces as LET increases.

◦ High LET radiation preferentially increase the repair independent damage in the cells.

◦ High LET radiation also leads to reduced variability in the cell cycle dependant radiosensitivity of cells.

Cellular damage may occur directly when the radiation interacts with the atom directly ( e.g. neutrons) or indirectly when interaction occurs by secondary electrons (e.g. photon beams).

Electrons produced by the ionizing events lead to further ionizations as they move inside biological material --> these lead to the formation of highly reactive free radicals like OH-, H- radicals which in turn lead to chemical changes by breaking chemical bonds.

Some of these reactions are potentially damaging to the cell, others effectively inactivate the radicals.

The reactions that most commonly lead to cell damage usually occur at the level of the DNA although they may occur at the level of cell membranes, proteins etc.

The main effect of radiation is to cause ionisation of the atoms in the absorbing medium.

Thus, when cells are irradiated, it is likely that ionisation of one or more of the atoms on some of the DNA molecules will occur.

This can lead to a number of consequences for the affected molecule. These effects include ◦ breakage of the chains of molecules comprising the DNA, and ◦ breakage of the links between chains.

The human body (about 60%) is made up of water, and the ionizing effects of radiation on water can lead to an indirect attack on DNA. The direct attack of radiation on the structure of DNA is not the only means by which radiation can affect cells.

The radiation produces H2O2

after reaction with Water. Hydrogen peroxide is a chemically active and is capable of reacting with DNA to damage cells and the genetic information contained therein.

The three major forms of interaction of radiation with matter, which are of clinical importance in radiotherapy are:

1. Compton effect.2. Photoelectric effect.3. Pair production.

Out of these, the Compton effect is the most important in modern-day megavoltage radiation therapy.

The reduced scattering suffered by high-energy radiation as well as the almost homogeneous tissue dosage is primarily due to the Compton effect.

The photoelectric effect is of primary importance in diagnostic radiology and has only historical importance in present day radiotherapy.

Despite several decades of research, photon-beam still constitute the main therapeutic modality in radiotherapy, because of several unresolved technical problems with the use of particulate radiation.

Any Questions?