Radiation Protection in Radiotherapy

1. Radiation Protection inRadiotherapy IAEA Training Material on Radiation Protection in Radiotherapy Part 2 Radiation Physics

2. 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

3. 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

4. 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

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

6. 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

7. 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

8. Identification of an Isotope Atom Electrons Nucleus Part 2, lecture 1: General radiation physics

9. Henri Becquerel (1852-1908) Discovered radioactivity in 1896 Part 2, lecture 1: General radiation physics

10. 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

11. 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

12. 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

13. Half life - logarithmic plot Part 2, lecture 1: General radiation physics

14. 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

15. Alpha decay Part 2, lecture 1: General radiation physics

16. Beta decay Part 2, lecture 1: General radiation physics

17. Gamma transition Excited state Part 2, lecture 1: General radiation physics

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. X Ray Tube for low and medium X Ray production Part 2, lecture 1: General radiation physics

26. Megavoltage X Ray linac Part 2, lecture 1: General radiation physics

27. 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

28. 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

29. 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

30. 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

31. The resulting X Ray spectrum Characteristic X Rays Bremsstrahlung Spectrum after filtration Maximum electron energy Part 2, lecture 1: General radiation physics

32. The effect of additional filtration Part 2, lecture 1: General radiation physics

33. 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

34. 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

35. Types of Radiation Ionization Events Part 2, lecture 1: General radiation physics

36. Which one is indirectly ionizing ? Ionization Events Part 2, lecture 1: General radiation physics

37. Comparison of depth dose characteristics Part 2, lecture 1: General radiation physics

38. …photons are most commonly used Part 2, lecture 1: General radiation physics

39. Photons are part of the electromagnetic spectrum Part 2, lecture 1: General radiation physics

40. Photons are part of the electromagnetic spectrum Enough energy to cause ionization Part 2, lecture 1: General radiation physics

41. Photon Interactions Part 2, lecture 1: General radiation physics

42. Albert Einstein • Explanation of the photo-effect Part 2, lecture 1: General radiation physics

43. Part 2, lecture 1: General radiation physics

44. Variation of photon interaction coefficient with energy Therapeutic X Ray range Part 2, lecture 1: General radiation physics

45. Variation of attenuation with atomic number Part 2, lecture 1: General radiation physics

46. Variation of attenuation with atomic number Part 2, lecture 1: General radiation physics

47. 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

48. Secondary and tertiary particles in a megavoltage photon beam Part 2, lecture 1: General radiation physics

49. 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

50. 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