Interaction of radiation with matter in radiation oncology

1. Interaction of Radiation
with Matter
1

2. INTRODUCTION
• Objective:
• To understand the important interaction processes
between radiation and matter.
Two basic entities need to be discussed before
dealing with interaction of radiation with matter:
 RADIATION
 MATTER
2

3. RADIATION
• Definition: “It is a form of energy which can be emitted
and propagated through the space or a medium by
different modes of energy transfer and deposition.”
Types of
Radiation
Electromagnetic Particulate
3

4. ELECTROMAGNETIC RADIATION
E and H are the peak amplitudes of electric and magnetic
fields respectively.
ELECTROMAGNETIC RADIATION
Electromagnetic Radiation consists of oscillating electric
and magnetic fields which are right angles to each other and to the
direction of the energy propagation
Electromagnetic radiations are:
• Radio waves
• Infrared radiations
• Visible-light
• Ultra violet radiations
• X-rays
• Gamma rays
• Cosmic rays
4

5. ELECTROMAGNETIC RADIATION
• Electromagnetic radiation:
 Wave model
 Quantum model
Wave : Continuous flow of energy
c = υ λ
c- velocity, υ- frequency, λ- wavelength
Photon : Packages of energy (quanta )
E= h υ
E- energy , h – Planck’s constant(6.63x10 -³³js), υ- frequency
E= h c/ λ
5

6. Common Properties
 Electromagnetic waves travel in straight line.
 Travel at speed of light (c) in a vacuum (or air)
c= 3x 1010 cm/s or 3x10 8 m/s
 Transfer energy from place to place in quanta
 Intensity is reduced (attenuation) while passing through a
medium because of absorption and scattering processes.
 In free space, all obey inverse square law
i.e,
“ Intensity from a point source is inversely proportional
to square of distance at which intensity is measured.”
ELECTROMAGNETIC RADIATION
6

7. Ionizing
ionizes [strips electrons
from] atoms
Non-Ionizing
many other modes of
interaction
Electro Magnetic Spectrum
7

8. ELECTROMAGNETIC
SPECTRUM
E= h c/ λ or E= h υ
8

9. CLASSIFICATION OF RADIATION
– Depending upon ionization property:
radiation can be classified as
1.Ionizing: removes electrons from atoms
Directly ionizing (Alpha and Beta)
Indirectly ionizing (Gamma, X-rays &
neutrons)
2.Non-ionizing: can't remove electrons from atoms
infrared, visible, microwaves, radar, radio waves,
lasers
9

10. MATTER
• Matter can simply be described in terms of the atomic
number of the constituent elements
• Matter is composed of elements
• ATOMS : a particle of an element
• An atom consists of a positively charged nucleus
surrounded by a cloud of negatively charged electrons
• An atom is specified by the formula A
ZX, where
 X is the symbol for the element,
 A is the mass number (number of protons + neutrons),
 Z is the atomic number (number of protons).
10

11. THE ATOMIC STRUCTURE
Central nucleus
(protons and neutrons)
surrounded by
orbiting electron
Dimension :
Atom 10-10m
Nucleus 10-15m
11

12. PLANETORY MODEL OF ATOM
• NIELS BOHR – in 1913
Hydrogen atom – extended to
multi-electron atom
• Energy levels- K,L ,M…….. so forth from nucleus
• No. of electron 2 x n2
n – integer specific to each shell
( principal quantum no. )
• Energy: released - electron moves to closer orbit
: required - to move to higher orbit
12

13. Fig : Bohr’s model of the
atom
Fig : Energy level diagram
 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.
13

14. The process by which one or more electrons are removed from an
atom and a neutral atom acquires a positive or negative charge
IONIZATION
electron is
stripped from
atom
-
-
-
-
The neutral atom
gains a + charge
= an ion
+
+
Alpha Particle
14

15. IONIZATION
Formation of a charged and reactive atom
-
-
-
-
The neutral absorber atom
acquires a positive charge
Beta particle
-
Colliding
coulombic fields
Ejected electron
15

16. Excitation
• It is a process by which the orbital electrons
of an atom are raised to a higher energy
level.
• Inner shell electrons are imparted sufficient
energy to “jump up” to higher energy level.
• Electron then immediately “jumps down” to
its original shell to fill the vacancy and in
this process , excess energy is shed as
electromagnetic radiation
• This EM Radiation is characteristic of a
given element ( difference in shell energy
levels) and is called characteristic
radiation
16

17. INTERACTION OF RADIATION WITH
MATTER
INTRODUCTION
 Study of these interactions is medically
important because
1. Mechanism of cell injury caused by
radiation exposure.
2. Radiation protection: shielding requirements.
3. Detection of radiation.
17

18. Contd…..
• Unaffected by the chemical or physical state of
the elements.
• During the passage of radiation through matter,
radiation is either absorbed or scattered.
• Effect of radiation on matter depends on amount
of energy absorbed by the matter.
• The specific mechanisms which are responsible
for either absorption or scattering vary with type
of radiation.
18

19. INTERACTION OF RADIATION WITH MATTER
contd…
• When photon passes through matter , it may be
1 . Transmitted unchanged
2. Deflected from its original path to a new
direction with unchanged energy
3 . Deflected and lose some energy
4 . Disappear altogether
Ref: Fundamental Physics of Radiology by W. J. Meredith & Messey
19

20. Process Definition
Attenuation Reduction in intensity of radiation 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 it contributes
to both attenuation and absorption
Transmission Any photon, which does not suffer the above processes is
transmitted.
20

21. • When 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.
Attenuation
Fig : Semilog plot showing exponential
attenuation of a monoenergetic photon
beam.
21

22. ATTENUATION
• The greater the thickness of material , the
greater the attenuation.
• The greater the atomic number and/or the
density of the material, the greater the
attenuation produced by any given thickness.
• The greater the photon energy , the smaller the
attenuation produced by a given thickness of a
particular material.
Ref: Fundamental Physics of Radiology by W. J. Meredith & Messey
22

23. Half-value-layer (HVL)
• Half-value-layer (HVL)- The thickness of the absorber material required
to decrease (attenuate) the intensity of a monoenergetic photon-beam to
half of its original value.
• This shows the quality or the penetrating power of an x-ray beam.
2nd HVL
1st HVL
First layer will decrease the intensity of the incident beam to half of its original value,
which gets further reduced to its 1/4th by the next layer of the material. 23

24. • 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 Coefficients
24

25. Attenuation of a photon beam by an absorbing material is caused by five
major types of interactions :
Attenuation
Coherent
scattering
Photoelectric
effect
Compton
effect
Pair
production
Photo
disintegration
Processes causing Attenuation
25

26. Basically FIVE main processes which describe the
interaction of x-ray and gamma ray through matter.
These are :
1. Elastic or Classical or Unmodified or
Thomson or Rayleigh or Coherent
scattering.
2. In-elastic or Compton or Modified or
Incoherent scattering.
3. Photoelectric effect.
4. Pair production.
5. Photo disintegration
26

27. Matter
Photo electric
effect
Compton
Scatter
Pair
Production
Matter
Ionization
X-Rays Chemical Effects
Biological Effects
Excitation Heat
No Biological
Effects
High Speed Electrons
Photon
27

28. 1. CLASSICAL/ELASTIC
SCATTERING
• Also known as coherent scattering, unmodified,
Thomson, classical, Rayleigh scattering
• Explained by considering radiation as a waves rather
than photon
• Radiation Interaction with bound electron causes
oscillation of electron which reradiates energy and
scatters x rays with same frequency in all directions
• Radiation is deflected with same frequency (energy)
at small angles.
• The scattered photon has the same wavelength as the
incident photon.
• No energy is changed in to electronic motion
• No energy is absorbed in the medium
Ref: Fundamental Physics of Radiology by W. J. Meredith & Messey
28

29. classical scattering or Rayleigh scattering
or coherent scattering
Diagram illustrating the process of coherent scattering. The scattered photon has the
same wavelength as the incident photon. No energy is transferred.( No attenuation and
No absorption)
Ref: Fundamental Physics of Radiology by W. J. Meredith & Messey29

30. CLASSICAL/ELASTIC SCATTERING Contd…
• This process involves bound electron, coherent scattering
occurs more in high atomic number materials and with
low energy radiations.
• MASS ATTENUATION CO-EFFICIENT- Measure of the
probability of this process,
Directly proportional to Z2.
Inversely proportional to radiation energy (E)
 This process is of academic interest in radiotherapy
 It is important in X-ray crystallography: to know about
the structure of materials
Ref: Fundamental Physics of Radiology by W. J. Meredith & Messey30

31. 2. COMPTON SCATTERING
• Also known as incoherent scattering, modified
scattering
• Compton process involves transfer of a part of the
energy of the incoming photon to a “free electron”.
• Electron receives some energy and ejected at an angle
 and photon with reduced energy (increased
wavelength) scattered at an angle .
• Since the Compton process involves these free
electrons, the process is independent of the atomic
number of the medium in which the interaction takes
place.
Ref: Fundamental Physics of Radiology by W. J. Meredith & Messey31

32. COMPTON SCATTERING
-
--
Incoming photon
Collides with
electron
-
-
--
Electron is
ejected from atom
-
Scattered Photon
32

33.  If the angle by which the electron is ejected is θ and the
angle by which the photon is scattered is Φ, then the
following formula describes the change in the wavelength
(δλ)of the photon:
λ2 – λ1 = δλ = 0.024 ( 1- cos θ) Å
COMPTON SCATTERING contd…
Ref: Fundamental Physics of Radiology by W. J. Meredith & Messey
33

34. COMPTON SCATTERING contd…
1. Direct hit:
• Photon makes a direct hit with the electron,
• Photon scattered backward ( = 180 degrees)
• Scattered photon left with minimum energy
• Electron travel forward (=0degrees)
• Electron receive maximum energy (E max) .
2. Grazing hit:
• Photon makes a grazing hit with the electron
• Photon scattered forward direction ( = 0 degrees)
• Electron at right angle( = 90 degrees)
3. 90-degree photon scatter
Photon is scattered at right angles to its original direction (φ =
90 degrees).
34

35. COMPTON SCATTERING contd…
salient features…
1. The fraction of the energy absorbed in a
collision increases with increase in energy of
the incident photon
2. The fraction of the energy scattered is large for
low energy photons and very low for high
energy photon.
3. Compton effect decreases with increase in energy
of the incident photon
Ref: Fundamental Physics of Radiology by W. J. Meredith & Messey 35

36. COMPTON SCATTERING contd…
4. In soft tissue the range of 100 keV to 10 MeV,
Compton absorption is much more prominent than
photoelectric or pair production process.
5. As the energy of the incident photon energy
increased, the electron will be ejected in more in
forward direction and will carry the maximum
energy.
6. 75% of radiation damage is caused by ejected
electron from the orbit and 25% of radiation
damage is caused by free radical.
36

37. 3. Photoelectric effect
• In this process, the photon imparts all its energy to an
orbital electron of an atom of the medium and causes
ejection of electron (photoelectron) from its orbit.
• Vacancy created in shell filled by outer electron with
emission of characteristic x ray
• Absorption of x ray internally produces mono energetic
AUGER electron.
• A photon of energy hν will release an electron with
kinetic energy
KE = hν– EB Where EB is the binding energy of the
electron for that particular orbit.
37

38. PHOTOELECTRIC EFFECT
38

39.  Photoelectric process involves bound electron.
 The probability of ejection of an electron is maximum when
the photon energy is just higher than the binding energy of the
electron. (Photon energy h > electron binding energy EB)
 The mass photoelectric attenuation co-efficient
Directly proportional to Z3.
Inversely proportional to E3
where, Z - Atomic no. E - Photon energy
 The probability of interaction decreases as h increases.
 High Z materials are strong X Ray absorber
 As the photon energy increases there is greater probability for
photoelectron to be ejected in the forward direction.
 Photoelectric effect is predominant up to 60keV.
PHOTOELECTRIC EFFECT CONTD….
39

40. • 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.
PHOTOELECTRIC EFFECT
40

41. PAIR PRODUCTION
• Pair production is the conversion of a photon into a pair of
positive and negative charges and this interaction occur in the
nuclear field.
• Since the creation of the pair requires a minimum energy of 1.02
MeV
(which is twice the rest mass energy according to the mass energy
relationship E = mC2 ), photon must possesses at least 1.02 MeV .
• Energy in excess of 1.02 MeV is shared equally in the form of
kinetic energy by the pair formed.
41

42. Contd….
• The positron having lost its kinetic energy combined
with an electron giving rise to annihilation radiation
normally in the form of two photons each with 0.51
MeV moving in opposite directions.
• The attenuation coefficient varies with Z2 per atom.
• This process increases with increase in energy of the
incoming photon.
42

43. PAIR PRODUCTION
43

44. Annihilation radiation
44

45. PRINCIPAL MODES OF
INTERACTION
45

46. 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.
Relative Importance OF P.E. (τ), Compton (σ) And Pair
production (π ) processes in Water
46

47.  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.
Photo Disintegration Reaction
47

48. CLINICAL IMPORTANCE
• CLASSICAL SCATTERING
Academic interest in radiotherapy
 Important in X-ray crystallography
• Photoelectric effect
 Diagnostic radiology procedures
• Compton effect
 Radio therapy procedures
• Pair production
 PET scan
48

49. INTERACTION OF PARTICLES
WITH MATTER
Charged particles (electrons, protons, α particles)
interact principally by:
 Ionization
 Excitation.
49

50. ELECTRONS
• Interaction can be
 Elastic
 Inelastic
– Elastic collisions occur with either atomic electrons or
with atomic nuclei
- characterized by change in only direction
with no loss of energy
– Inelastic collisions occur with
- atomic electrons results in ionization and
excitation of atoms
-atomic nuclei results in production of
BREMSSTRAHLUNG x rays
(braking radiation)
50

51. Contd…
• Electrons ejected by ionization can acquire sufficient
energy to cause ionization and ejected electrons are called
SECONDARY ELECTRONS or DELTA RAYS.
• Typical energy loss in tissue for a therapeutic electron
beam , averaged over its entire range is about 2
MeV/cm of water.
• As electrons are very lighter than atomic nuclei, electron
can loss large fraction of energy in a single process and
deflected by very large angles .
• Even if the electron is monoenergetic when on first
impinging , large variation occur among moving
electrons( range straggling)
51

52. Electrons contd.....
• Probability of bremsstrahlung production per atom is proportional to the
square of Z of the absorber
• Energy emission via bremsstrahlung varies inversely with square of mass
of incident particle
52

53. PROTONS AND HEAVY IONS
• Protons traverse straight path, slowing down continuously by
interactions with atomic electrons and atomic nuclei .
• Results in depth dose characteristic - shows constant absorbed
dose value over most of the beam range until near the end sharp
increase in the dose occur (BRAGG PEAK)
• The RBE of proton beam is similar to photon and electron beams.
• Because of the Bragg peak effect and minimal scattering, protons
and heavier charged particle have the ability to concentrate dose
inside the target volume and minimize dose to surrounding
normal tissues.
• The rate of energy loss or stopping power of charged particles is
proportional to the square of the particle charge
inversely proportional to the square of its velocity
53

54. BRAGG PEAK
Depth dose distribution for various heavy particle beams with modulated Bragg
peak at a depth of 10 cm and normalized at the peak center. (From Raju MR.
Heavy Particle Radiotherapy. New York: Academic Press, 1980.) 54

55. NEUTRONS
• Like x-rays and γ rays, neutrons are indirectly ionizing.
• Neutrons interact basically by two processes:
(a) recoiling protons from hydrogen and recoiling heavy
nuclei from other elements
(b) nuclear disintegrations.
• Exponentially attenuated by matter.
• Interactions are primarily nuclear and include elastic
scattering .
• Neutron interactions result in recoil protons and
charged nuclear fragments which have relatively low
energy.
55

56. Lead is an efficient absorber of x-rays but not of
neutrons.
The most efficient absorber of neutrons is a hydrogenous
material such as water, paraffin wax, and polyethylene
56

57. Conclusion
Rayleigh
Scattering
Compton
effect
Photoelectric
effect
Pair
production
Photon
interaction
With bound
electrons
With free
electrons
With whole atom
(bound electron)
With nuclear
Coulomb field
Mode of photon
interaction
Photon
scattered
Photon
scattered
Photon
disappears
Photon
Disappears
Energy
dependence
Decreases
with energy
Increases with
Energy
Threshold No No No 2mec²
Linear
attenuation
coefficient
Particles
released
None Compton
(recoil)
electron
Photoelectron Electron–
positron pair
57

58. Conclusion
Mass
coefficient
dependence
on Z
Independent
Subsequent
effect
None Characteristic
X ray,
Auger effect
Characteristic
X ray,
Auger effect
Annihilation
radiation
Significant
energy region
for water
<20 keV 20 keV–
10 MeV
<20 keV >10 MeV
58

59. Thank you for your attention
Any Questions?
59