Radiation Protection in Radiotherapy Part 2 Radiation Physics IAEA Training Material on Radiation Protection in Radiotherapy

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics2 Background l Radiation generation, transport and interaction with matter are physical processes: n While radiation cannot be seen or felt, it can be well described and physically quantified n It can be accurately determined using appropriate experimental set-ups

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics3 Objectives of the Module l To be familiar with different types of ionizing radiation l To understand the most important interaction processes between radiation and matter l To be able to use and understand all basic radiation quantities l To have a basic understanding of the means of radiation detection

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics4 Contents l Lecture 1: General n Radioactivity n Types of ionizing radiation n Interaction of radiation with matter n Radiation quantities and units l Lecture 2: Equipment n Basic means of radiation detection

Radiation Protection in Radiotherapy Part 2 Radiation Physics Lecture 1: General Radiation Physics IAEA Training Material on Radiation Protection in Radiotherapy

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics6 Radiation = Ionizing Radiation l Sufficient energy to ionize atoms (eject an electron or add an additional one) l This leaves a charged ion. l The ion will upset chemical bonds l If this affects critical molecules such as DNA (either directly or indirectly) this can result in cell damage, mutation or death.

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics7 Contents 1. Radioactivity 2. Types of ionizing radiation 3. Interaction of radiation with matter 4. Radiation quantities and units

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics8 Identification of an Isotope Nucleus Atom Electrons

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics9 Henri Becquerel ( ) Discovered radioactivity in 1896

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics10 1. Radioactivity l A property of nuclei l 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

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics11 Half life t 1/2 l Describes how fast a particular nucleus transforms l The time it takes for half the amount of a radioactive material to transform (often also referred to as decay)

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics12 A(t) = A(0) exp(-t ln2 / t 1/2 ) l A(t) activity at time t l A(0) original activity at time 0 l t time l t 1/2 half life

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics13 Half life - logarithmic plot

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics14 Types of radioactivity l Alpha particles (Helium nuclei) - “heavy”, dual positive charge, strongly interacting with matter l Beta particles/radiation (electrons) - light particle, loosely interacting, still finite range l Gamma radiation (photons)

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics15 Alpha decay

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics16 Beta decay

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics17 Gamma transition Excited state

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics18 Radioactivity l More information on radioactive isotopes used in radiotherapy are provided in part 6 of the present course l More information on radioactivity is also provided in the companion course on nuclear medicine - in radiotherapy typically the radiation itself is the main consideration...

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics19 2. Ionizing Radiation l Radioactivity is ONE source of ionizing radiation l Deposits an amount of energy in matter which is sufficient to cause the breaking of chemical bonds l Wave and particle descriptions are both used and correct representations l One radiation particle often deposits energy at multiple sites - either directly or via the creation of other particles

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics20 Types of Radiation (1) l X Rays and gamma rays = photons l electrons and beta particles - negative charge l neutrons l protons - positive charge l alpha particles and heavy charged particles

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics21 Types of Radiation (2) l X Rays and gamma rays = photons l electrons and beta particles - negative charge l neutrons l protons - positive charge l alpha particles and heavy charged particles Photons and electrons are the most important types of radiation in Radiotherapy

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics22 Photons l Gamma-rays: monoenergetic (one or more lines) l X Rays: a spectrum l The difference lies in the way of production: n Gamma in nucleus n X Ray in atomic shell CW Roentgen, discoverer of X-rays

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics23 X Ray production l High energy electrons hit a (metallic) target where part of their energy is converted into radiation target electrons X Rays Low to medium energy (10-400keV) High > 1MeV energy

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics24 Issues with X Ray production l 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 target Low to medium energy (10-400keV) High > 1MeV energy

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics25 X Ray Tube for low and medium X Ray production

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics26 Megavoltage X Ray linac

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics27 Issues with X Ray production l Angular distribution: high energy X Rays are mainly forward directed, while low energy X Rays are primarily emitted perpendicular to the incident electron beam l 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

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics28 Types of X Ray production l Characteristic X Rays: n 1 The incoming electron knocks out an inner shell atomic electron n 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 1 2

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics29 Types of X Ray production l Bremsstrahlung: The incoming electron is deflected in the atomic shell and decelerated. The energy difference is emitted as an X Ray

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics30 Bremsstrahlung production l The higher the atomic number of the X Ray target, the higher the yield l The higher the incident electron energy, the higher the probability of X Ray production l At any electron energy, the probability of generating X Rays decreases with increasing X Ray energy

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics31 The resulting X Ray spectrum Characteristic X Rays Bremsstrahlung Spectrum after filtration Maximum electron energy

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics32 The effect of additional filtration

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics33 Types of Radiation (3) l Directly ionizing radiation - energy is deposited by the particle directly in matter (electrons, protons) l Indirectly ionizing radiation - primary particle transfers energy to secondary particle which in turn causes ionization events (photons, neutrons)

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics34 3. Interaction of radiation with matter l Determines penetration (how much radiation reaches a target) l Determines dose deposited in the target ?

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics35 Types of Radiation Ionization Events

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics36 Which one is indirectly ionizing ? Ionization Events

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics37 Comparison of depth dose characteristics

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics38 …photons are most commonly used

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics39 Photons are part of the electromagnetic spectrum

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics40 Photons are part of the electromagnetic spectrum Enough energy to cause ionization

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics41 Photon Interactions

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics42 Albert Einstein l Explanation of the photo-effect

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics43

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics44 Variation of photon interaction coefficient with energy Therapeutic X Ray range

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics45 Variation of attenuation with atomic number

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics46 Variation of attenuation with atomic number

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics47 Consequences l Lead shielding very efficient at low photon energies (diagnostics) l In general, photons are difficult to attenuate, in particular in the megavoltage range used for therapy l Megavoltage photons are less suitable for imaging

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics48 Secondary and tertiary particles in a megavoltage photon beam

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics49 Electron interaction in matter l 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 l Bremsstrahlung (= “braking radiation”). The electron loses energy in form of X Rays as it is deflecting around nuclei

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics50 Bremsstrahlung l Most effective for electrons of very high energy in materials of high atomic number (metals). l The production process of X Rays in the first place… target X Rays electrons

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics51 Electron interactions

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics52 Electron interactions Tertiary photons from Bremsstrahlung

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics53 Photons Electrons l Exponential attenuation l Indirectly ionizing l Finite range l Directly ionizing

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics54 From radiation to energy deposition in a photon beam

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics55 4. Radiation quantities and units l Need to quantify radiation effects to n determine and quantify risks n determine the likelihood of benefit (cancer cure or palliation) n to weigh risk and benefit n to optimize radiotherapy approaches n to make informed decisions

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics56 Characterization of radiation Source Energy Deposition First Interaction Transport

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics57 Physical quantities which can be measured l At the source: Activity, mA, kV l On the flight: Flux, fluence l At the first interaction point: Kinetic Energy Released in Matter (KERMA) l In matter: Absorbed dose

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics58 Activity n The ‘amount’ of a radionuclide n SI unit is the Becquerel (Bq) - one nuclear transformation per second n Old unit is the Curie (Ci) 1 Ci = 37 x 10 9 Bq = 37 GBq

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics59 1 Bq is a small quantity l 40-Potassium in every person > 1000Bq l Many radioactive sources are > 100,000Bq l Radioactive sources in radiotherapy often > 100,000,000Bq

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics60 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

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics61 Exposure l Number of charges created by radiation in air l Relatively easy to determine l Measured in coulomb per kilogram (C/kg) - old unit roentgen (R) 1 R = 2.58 x C/kg

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics62 Absorbed Dose l Energy deposited in matter l D = E/m (1 Gy = 1 J/kg) l The unit related to effects in matter l Not necessarily a straight forward relationship to the intensity of the radiation beam

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics63 From Exposure to Dose EXPOSURE l only defined in air l ‘first impact’ quantity DOSE l can be defined in any medium using stopping power ratios l can be derived from exposure using W/e

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics64 1 Gy is a relatively large quantity l Radiotherapy doses > 1Gy l Dose from diagnostic radiology typically < 0.001Gy l Annual background radiation due to natural radiation (terrestrial, cosmic, due to internal radioactivity, Radon,…) about 0.002Gy

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics65 Fractions & Prefixes (Dose) Fraction Prefix Abbreviation 1 -Gy 1 -Gy 1/1000milli (m)mGy 1/1,000,000micro (  )  Gy

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics66 Summary l In radiotherapy, photons (X Rays and gamma rays) and electrons are the most important radiation types l There are several different interaction processes possible for photons - all important ones transfer energy to an electron which deposits the energy in tissue. l Absorbed dose is defined as energy deposited in matter and measured in Gray

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics67 Where to Get More Information l Medical physicists l Textbooks: s sKhan F. The physics of radiation therapy s sMetcalfe P.; Kron T.; Hoban P. The physics of radiotherapy X-rays from linear accelerators s sCember H. Introduction to health physics s sWilliams J; Thwaites D. Radiotherapy Physics

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics68 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

Any questions?

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

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics71 Simulator (kV) and portal image (MV) of the same anatomical site (prostate) l Reference simulator film l Check portal film

Radiation Protection in RadiotherapyPart 2, lecture 1: General radiation physics72 Acknowledgment l Robin Hill, Liverpool Hospital, Sydney