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Radiation Protection in Radiotherapy. IAEA Training Material on Radiation Protection in Radiotherapy. Part 2 Radiation Physics. Background. Radiation generation, transport and interaction with matter are physical processes:

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radiation protection in radiotherapy

Radiation Protection inRadiotherapy

IAEA Training Material on Radiation Protection in Radiotherapy

Part 2

Radiation Physics

background
Background
  • Radiation generation, transport and interaction with matter are physical processes:
    • While radiation cannot be seen or felt, it can be well described and physically quantified
    • It can be accurately determined using appropriate experimental set-ups

Part 2, lecture 1: General radiation physics

objectives of the module
Objectives of the Module
  • To be familiar with different types of ionizing radiation
  • To understand the most important interaction processes between radiation and matter
  • To be able to use and understand all basic radiation quantities
  • To have a basic understanding of the means of radiation detection

Part 2, lecture 1: General radiation physics

contents
Contents
  • Lecture 1: General
    • Radioactivity
    • Types of ionizing radiation
    • Interaction of radiation with matter
    • Radiation quantities and units
  • Lecture 2: Equipment
    • Basic means of radiation detection

Part 2, lecture 1: General radiation physics

radiation protection in radiotherapy1

Radiation Protection inRadiotherapy

IAEA Training Material on Radiation Protection in Radiotherapy

Part 2

Radiation Physics

Lecture 1: General Radiation Physics

radiation ionizing radiation
Radiation = Ionizing Radiation
  • Sufficient energy to ionize atoms (eject an electron or add an additional one)
  • This leaves a charged ion.
  • The ion will upset chemical bonds
  • If this affects critical molecules such as DNA (either directly or indirectly) this can result in cell damage, mutation or death.

Part 2, lecture 1: General radiation physics

contents1
Contents

1. Radioactivity

2. Types of ionizing radiation

3. Interaction of radiation with matter

4. Radiation quantities and units

Part 2, lecture 1: General radiation physics

identification of an isotope
Identification of an Isotope

Atom

Electrons

Nucleus

Part 2, lecture 1: General radiation physics

henri becquerel 1852 1908
Henri Becquerel (1852-1908)

Discovered radioactivity in 1896

Part 2, lecture 1: General radiation physics

1 radioactivity
1. Radioactivity
  • A property of nuclei
  • Due to inherent physical properties, a nucleus may be not stable and likely to undergo a nuclear transformation. This process can be fast (short half life) or slow (long half life). In any case, the time of transformation cannot be predicted for an individual nucleus - it is a random event which can only be adequately described using statistics

Part 2, lecture 1: General radiation physics

half life t 1 2
Half life t1/2
  • Describes how fast a particular nucleus transforms
  • The time it takes for half the amount of a radioactive material to transform (often also referred to as decay)

Part 2, lecture 1: General radiation physics

a t a 0 exp t ln2 t 1 2
A(t) = A(0) exp(-t ln2 / t1/2)
  • A(t) activity at time t
  • A(0) original activity at time 0
  • t time
  • t1/2half life

Part 2, lecture 1: General radiation physics

half life logarithmic plot
Half life - logarithmic plot

Part 2, lecture 1: General radiation physics

types of radioactivity
Types of radioactivity
  • Alpha particles (Helium nuclei) - “heavy”, dual positive charge, strongly interacting with matter
  • Beta particles/radiation (electrons) - light particle, loosely interacting, still finite range
  • Gamma radiation (photons)

Part 2, lecture 1: General radiation physics

alpha decay
Alpha decay

Part 2, lecture 1: General radiation physics

beta decay
Beta decay

Part 2, lecture 1: General radiation physics

gamma transition
Gamma transition

Excited state

Part 2, lecture 1: General radiation physics

radioactivity
Radioactivity
  • More information on radioactive isotopes used in radiotherapy are provided in part 6 of the present course
  • More information on radioactivity is also provided in the companion course on nuclear medicine - in radiotherapy typically the radiation itself is the main consideration...

Part 2, lecture 1: General radiation physics

2 ionizing radiation
2. Ionizing Radiation
  • Radioactivity is ONE source of ionizing radiation
  • Deposits an amount of energy in matter which is sufficient to cause the breaking of chemical bonds
  • Wave and particle descriptions are both used and correct representations
  • One radiation particle often deposits energy at multiple sites - either directly or via the creation of other particles

Part 2, lecture 1: General radiation physics

types of radiation 1
Types of Radiation (1)
  • X Rays and gamma rays = photons
  • electrons and beta particles - negative charge
  • neutrons
  • protons - positive charge
  • alpha particles and heavy charged particles

Part 2, lecture 1: General radiation physics

types of radiation 2
Types of Radiation (2)
  • X Rays and gamma rays = photons
  • electrons and beta particles - negative charge
  • neutrons
  • protons - positive charge
  • alpha particles and heavy charged particles

Photons and electrons are

the most important types of

radiation in Radiotherapy

Part 2, lecture 1: General radiation physics

photons
Photons
  • Gamma-rays: monoenergetic (one or more lines)
  • X Rays: a spectrum
  • The difference lies in the way of production:
    • Gamma in nucleus
    • X Ray in atomic shell

CW Roentgen, discoverer of X-rays

Part 2, lecture 1: General radiation physics

x ray production
X Ray production
  • High energy electrons hit a (metallic) target where part of their energy is converted into radiation

electrons

Low to

medium

energy

(10-400keV)

High

> 1MeV

energy

target

X Rays

Part 2, lecture 1: General radiation physics

issues with x ray production
Issues with X Ray production
  • Angular distribution: high energy X Rays are mainly forward directed, while low energy X Rays are primarily emitted perpendicular to the incident electron beam - this is reflected in the target design

Low to

medium

energy

(10-400keV)

High

> 1MeV

energy

target

Part 2, lecture 1: General radiation physics

x ray tube for low and medium x ray production
X Ray Tube for low and medium X Ray production

Part 2, lecture 1: General radiation physics

megavoltage x ray linac
Megavoltage X Ray linac

Part 2, lecture 1: General radiation physics

issues with x ray production1
Issues with X Ray production
  • Angular distribution: high energy X Rays are mainly forward directed, while low energy X Rays are primarily emitted perpendicular to the incident electron beam
  • Efficiency of production: In general, the higher the energy, the more efficient the X Ray production - this means that at low energies most of the energy of the electron (>98%) is converted into heat - target cooling is essential

Part 2, lecture 1: General radiation physics

types of x ray production
Characteristic X Rays:

1 The incoming electron knocks out an inner shell atomic electron

2 An electron from a higher shell fills the vacancy and the energy difference is emitted as an X Ray of an energy characteristic for the transition

Types of X Ray production

2

1

Part 2, lecture 1: General radiation physics

types of x ray production1
Types of X Ray production
  • Bremsstrahlung: The incoming electron is deflected in the atomic shell and decelerated. The energy difference is emitted as an X Ray

Part 2, lecture 1: General radiation physics

bremsstrahlung production
Bremsstrahlung production
  • The higher the atomic number of the X Ray target, the higher the yield
  • The higher the incident electron energy, the higher the probability of X Ray production
  • At any electron energy, the probability of generating X Rays decreases with increasing X Ray energy

Part 2, lecture 1: General radiation physics

the resulting x ray spectrum
The resulting X Ray spectrum

Characteristic

X Rays

Bremsstrahlung

Spectrum after

filtration

Maximum electron energy

Part 2, lecture 1: General radiation physics

the effect of additional filtration
The effect of additional filtration

Part 2, lecture 1: General radiation physics

types of radiation 3
Types of Radiation (3)
  • Directly ionizing radiation - energy is deposited by the particle directly in matter (electrons, protons)
  • Indirectly ionizing radiation - primary particle transfers energy to secondary particle which in turn causes ionization events (photons, neutrons)

Part 2, lecture 1: General radiation physics

3 interaction of radiation with matter
3. Interaction of radiation with matter
  • Determines penetration (how much radiation reaches a target)
  • Determines dose deposited in the target

?

Part 2, lecture 1: General radiation physics

types of radiation
Types of Radiation

Ionization

Events

Part 2, lecture 1: General radiation physics

which one is indirectly ionizing
Which one is indirectly ionizing ?

Ionization

Events

Part 2, lecture 1: General radiation physics

comparison of depth dose characteristics
Comparison of depth dose characteristics

Part 2, lecture 1: General radiation physics

photons are most commonly used
…photons are most commonly used

Part 2, lecture 1: General radiation physics

photons are part of the electromagnetic spectrum
Photons are part of the electromagnetic spectrum

Part 2, lecture 1: General radiation physics

photons are part of the electromagnetic spectrum1
Photons are part of the electromagnetic spectrum

Enough energy

to cause ionization

Part 2, lecture 1: General radiation physics

photon interactions
Photon Interactions

Part 2, lecture 1: General radiation physics

albert einstein
Albert Einstein
  • Explanation of the photo-effect

Part 2, lecture 1: General radiation physics

variation of photon interaction coefficient with energy
Variation of photon interaction coefficient with energy

Therapeutic

X Ray range

Part 2, lecture 1: General radiation physics

variation of attenuation with atomic number
Variation of attenuation with atomic number

Part 2, lecture 1: General radiation physics

variation of attenuation with atomic number1
Variation of attenuation with atomic number

Part 2, lecture 1: General radiation physics

consequences
Consequences
  • Lead shielding very efficient at low photon energies (diagnostics)
  • In general, photons are difficult to attenuate, in particular in the megavoltage range used for therapy
  • Megavoltage photons are less suitable for imaging

Part 2, lecture 1: General radiation physics

secondary and tertiary particles in a megavoltage photon beam
Secondary and tertiary particles in a megavoltage photon beam

Part 2, lecture 1: General radiation physics

electron interaction in matter
Electron interaction in matter
  • Ionization events and excitation of atoms all along the electron path in matter. Individual energy depositions are small and a megavoltage electron may deposit energy at >10000 locations
  • Bremsstrahlung (= “braking radiation”). The electron loses energy in form of X Rays as it is deflecting around nuclei

Part 2, lecture 1: General radiation physics

bremsstrahlung
Bremsstrahlung
  • Most effective for electrons of very high energy in materials of high atomic number (metals).
  • The production process of X Rays in the first place…

electrons

X Rays

target

Part 2, lecture 1: General radiation physics

electron interactions
Electron interactions

Part 2, lecture 1: General radiation physics

electron interactions1
Electron interactions

Tertiary photons

from Bremsstrahlung

Part 2, lecture 1: General radiation physics

photons electrons
Exponential attenuation

Indirectly ionizing

Finite range

Directly ionizing

Photons Electrons

Part 2, lecture 1: General radiation physics

from radiation to energy deposition in a photon beam
From radiation to energy deposition in a photon beam

Part 2, lecture 1: General radiation physics

4 radiation quantities and units
4. Radiation quantities and units
  • Need to quantify radiation effects to
    • determine and quantify risks
    • determine the likelihood of benefit (cancer cure or palliation)
    • to weigh risk and benefit
    • to optimize radiotherapy approaches
    • to make informed decisions

Part 2, lecture 1: General radiation physics

characterization of radiation
Characterization of radiation

Energy

Deposition

Source

Transport

First

Interaction

Part 2, lecture 1: General radiation physics

physical quantities which can be measured
Physical quantities which can be measured
  • At the source: Activity, mA, kV
  • On the flight: Flux, fluence
  • At the first interaction point: Kinetic Energy Released in Matter (KERMA)
  • In matter: Absorbed dose

Part 2, lecture 1: General radiation physics

activity
Activity

The ‘amount’ of a radionuclide

SI unit is the Becquerel (Bq) - one nuclear transformation per second

Old unit is theCurie (Ci)

1 Ci = 37 x 109 Bq = 37 GBq

Part 2, lecture 1: General radiation physics

1 bq is a small quantity
1 Bq is a small quantity
  • 40-Potassium in every person > 1000Bq
  • Many radioactive sources are > 100,000Bq
  • Radioactive sources in radiotherapy often > 100,000,000Bq

Part 2, lecture 1: General radiation physics

multiple prefixes activity
Multiple & prefixes (Activity)

Multiple Prefix Abbreviation

1 -Bq

1,000,000Mega (M)MBq

1,000,000,000Giga (G)GBq

1,000,000,000,000Tera (T)TBq

Part 2, lecture 1: General radiation physics

exposure
Exposure
  • Number of charges created by radiation in air
  • Relatively easy to determine
  • Measured in coulomb per kilogram (C/kg) - old unit roentgen (R)

1 R = 2.58 x 10-4 C/kg

Part 2, lecture 1: General radiation physics

absorbed dose
Absorbed Dose
  • Energy deposited in matter
  • D = E/m (1 Gy = 1 J/kg)
  • The unit related to effects in matter
  • Not necessarily a straight forward relationship to the intensity of the radiation beam

Part 2, lecture 1: General radiation physics

from exposure to dose
EXPOSURE

only defined in air

‘first impact’ quantity

DOSE

can be defined in any medium using stopping power ratios

can be derived from exposure using W/e

From Exposure to Dose

Part 2, lecture 1: General radiation physics

1 gy is a relatively large quantity
1 Gy is a relatively large quantity
  • Radiotherapy doses > 1Gy
  • Dose from diagnostic radiology typically < 0.001Gy
  • Annual background radiation due to natural radiation (terrestrial, cosmic, due to internal radioactivity, Radon,…) about 0.002Gy

Part 2, lecture 1: General radiation physics

fractions prefixes dose
Fractions & Prefixes (Dose)

Fraction Prefix Abbreviation

1 - Gy

1/1000milli (m)mGy

1/1,000,000micro ()Gy

Part 2, lecture 1: General radiation physics

summary
Summary
  • In radiotherapy, photons (X Rays and gamma rays) and electrons are the most important radiation types
  • There are several different interaction processes possible for photons - all important ones transfer energy to an electron which deposits the energy in tissue.
  • Absorbed dose is defined as energy deposited in matter and measured in Gray

Part 2, lecture 1: General radiation physics

where to get more information
Where to Get More Information
  • Medical physicists
  • Textbooks:
      • Khan F. The physics of radiation therapy. 1994.
      • Metcalfe P.; Kron T.; Hoban P. The physics of radiotherapy X-rays from linear accelerators. 1997.
      • Cember H. Introduction to health physics. 1983
      • Williams J; Thwaites D. Radiotherapy Physics. 1993.

Part 2, lecture 1: General radiation physics

slide68

A note of caution:

Energy deposition in

matter is a random

event and the

definition of dose

breaks down for

small volumes (e.g.

a single cell). The

discipline of Micro-

dosimetry aims to

address this issue.

Adapted from Zaider 2000

Part 2, lecture 1: General radiation physics

question

Question:

What difference would one expect when using megavoltage photons for imaging of patients instead of the kilovoltage X Rays used in diagnostic radiology?

simulator kv and portal image mv of the same anatomical site prostate
Simulator (kV) and portal image (MV) of the same anatomical site (prostate)
  • Reference simulator film
  • Check portal film

Part 2, lecture 1: General radiation physics

acknowledgment
Acknowledgment
  • Robin Hill, Liverpool Hospital, Sydney

Part 2, lecture 1: General radiation physics