1. Radiobiology for the Radiologist, Hall, 7th edChapter 1. Physics and Chemistry of Radiation Absorption 2012.03.14 Dahoon Jung Korea Cancer Center Hospital

2. Overview Introduction Types of Ionizing Radiations Electromagnetic Radiations Particulate Radiations Absorption of X-rays Direct and Indirect Action of Radiation Absorption of Neutrons, Protons, and Heavy Ions Summary of Pertinent Conclusions

3. Introduction 1895, Wilhelm Conrad Rntgen, a new kind of ray X-rays

4. Introduction The first medical use of x-rays Lancet, Jan 23, 1896. To locate a piece of a knife in the backbone of a drunken sailor. The first recorded biologic effect of radiation was due to Becquerel. Left a radium container in his vest pocket. 2 weeks later, he found skin erythema and ulceration.

5. Introduction Radiobiology is the study of the action of ionizing radiations on living things.

6. Types of Ionizing Radiations The absorption of energy from radiation in biologic material may lead to excitation or to ionization. Ionizing radiation The energy dissipated per ionizing event is about 33 eV. Enough to break a strong chemical bond. Cf. The energy associated with a C=C bond is 4.9 eV. Classify ionizing radiations as either electromagnetic or particulate.

7. 1. Electromagnetic Radiations Most experiments with biologic systems have involved x- or -rays, two forms of electromagnetic radiation. Do not differ in nature or in properties. X-rays are produced extranuclearly. -rays are produced intranuclearly. Everything that is stated about x-rays in this chapter applies equally well to -rays.

8. 1. Electromagnetic Radiations X-rays as waves of electrical and magnetic energy. Velocity = c Wavelength = Frequency(the number of waves passing a fixed point per second) = = c c = 3 x 1010 cm/s

9. 1. Electromagnetic Radiations Radio waves, radar, radiant heat, and visible light are forms of electromagnetic radiation. same velocity, different frequency. different Energy. E =

10. 1. Electromagnetic Radiations X-rays as streams of photons, or packets of energy. Each energy packet equal to h. = 12.4/E(keV) for example, x-rays with wavelengths of 0.1 correspond to a photon E of 124 keV. The concept of x-rays being composed of photons is very important in radiobiology. Energy absorbed in living material is deposited unevenly in discrete packets. The individual packets, each of which is big enough to break a chemical bond and initiate the chain of events that culminates in a biologic change.

11. 1. Electromagnetic Radiations The critical difference between nonionizing and ionizing radiation is the size of the individual packets of energy, not the total energy involved. Total body dose of about 4 Gy of x-rays given to a human is lethal in about 50% of the individuals exposed. To 70kg, this dose is only about 67 cal. A temperature rise of 0.002 In the form of heat, drinking one sip of warm coffee. Electomagnetic radiations are usually considered ionizing if they have a photon energy in excess of 124 eV ( = 10-6 cm).

12. 2. Particulate Radiations Electrons, protons, -particles, neutrons, negative - mesons, and heavy charged ions. Electrons : small negatively charged Betatron , . or . Linear accelerator widely used for cancer therapy.

13. 2. Particulate Radiations Protons : Positively charged , relatively massive times (2000 greater than that of an electron), . cyclotron Major hazard to astronauts.

14. 2. Particulate Radiations -particles : nuclei of helium atoms. 2 protons and 2 neutrons. Net positivecharge. Also emitted during the decay heavy radionuclides(uranium and radium) of

15. 2. Particulate Radiations Neutrons : mass similar to that of protons, no electrical charge . Cannot be accelerated in an electrical device. Emitted as a by-product if heavy radioactive atoms undergo fission. Important component of space radiation and contribute significantly to the exposure of high- flying jetliners.

16. 2. Particulate Radiations Heavy charged particles : nuclei of elements, such as carbon, neon, argon, or even iron, that are positively charged because some or all of the planetary electrons have been stripped from them. Can be produced in only a few specialized facilities. Major hazard to astronauts on long missions.

17. Absorption of X-rays Radiation may be classified as directly or indirectly ionizing. All of the charged particles are directly ionizing. can disrupt the atomic structure of the absorber through which they pass directly and produce chemical and biologic changes.

18. Absorption of X-rays Electromagnetic radiations(x- and -rays) are indirectly ionizing. Do not produce chemical and biologic damage themselves. When absorbed in the material, they give up their energy to produce fast moving charged particles that in turn are able to produce damage.

19. Absorption of X-rays The process by which x-ray photons are absorbed depends on the energy of the photons and the chemical composition of the absorbing material. At radiotherapy, the Compton process dominates.

20. Absorption of X-rays Energy lost fraction vary from 0% to 80%. On a statistical basis, the net result is the production of several fast electrons. - break vital chemical bonds biologic damage.

21. Absorption of X-rays For photon energies, characteristic of diagnostic radiology, both Compton and photoelectric absorption process occur. In the PE process, the x-ray photon interacts with a bound electron in. KE = EB

22. Absorption of X-rays The vacancy left in the atomic shell as a result of the ejection of an electron. Filling by another electron from an outer shell or by a conduction electron from outside the atom. Decrease of potential energy. Characteristic electromagnetic radiation. Cf) in soft tissue, 0.5 kV.(of little biologic consequence)

23. Absorption of X-rays The Compton and photoelectric absorption process differ in several respects that are vital in the application of x- rays to diagnosis and therapy. The mass absorption coefficient for the Compton process is independent of the atomic number of the absorbing material. The mass absorption coefficient for photoelectric absorption varies rapidly with atomic number Z (Z3).

24. Absorption of X-rays High Z Low Z

25. Absorption of X-rays For radiotherapy, high-energy photons in the megavoltage range are preferred because the Compton process is overwhelmingly important. The absorbed dose is about the same in soft tissue, muscle, and bone. So that differential absorption in bone is avoided(the early days problem in RT).

26. Absorption of X-rays Although the differences among the various absorption processes are of practical importance in radiology, the consequences for radiobiology are minimal. Whether the PE or the Compton process, much of the absorbed energy is converted to the kinetic energy of a fast electron.

27. Direct and Indirect action of radiation The biologic effects of radiation result principally from damage to DNA, which is the critical target.

28. Direct and Indirect action of radiation Free radical(atom or molecule with an unpaired orbital electron) Easily modified by chemical means-either protectors or sensitizers- unlike direct action Dominant in high LET (neutrons or -particles) Radical cylinder

29. Direct and Indirect action of radiation(a) Production of hydroxyl radicals (OH) by ionizing radiation,(b) Cleavage reaction of the DNA-backbone after hydrogen abstraction at the C4-atomby the electrophilic, highly reactive hydroxyl radical.

30. Direct and Indirect action of radiation The period between the breakage of chemical bonds and the expression of the biologic effect may be hours, days, months, years, or generations, depending on the consequences involved. Cell killing hours to days later(when the damaged cell attempts to divide). Oncogenic may be delayed for 40 yrs. Mutation in a germ cell may not be expressed for many generations.

31. Absorption of Neutrons, Protons, and Heavy Ions In contrast to x-rays, neutrons interact not with the planetary electrons, but with the nuclei of the atoms that make up the tissue resulting in recoil protons.

32. Absorption of Neutrons, Protons, and Heavy Ions In the case of higher energy neutrons Spallation products

33. Absorption ofNeutrons, Protons, and Heavy Ions Protons interact with both planetary electrons to ionize the atoms, and also interact with the nuclei of atoms to produce heavier secondary particles.

34. Absorption of Neutrons, Protons, and Heavy Ions For neutrons or heavy ions, the direct action assumes greater importance. The indirect effect involving free radicals is most easily modified by chemical means. Radioprotective compounds are quite effective for x- and - rays, not for neutrons, -particles, or heavier ions.

35. Summary of Pertinent ConclusionsX- and -rays are indirectly ionizing; the first step in their absorption is theproduction of fast recoil electrons.Neutrons are also indirectly ionizing; the first step in their absorption is theproduction of fast recoil protons, -particles, and heavier nuclearfragments.Biologic effects of x-rays may be caused by direct action(the recoilelectron directly ionizes the target molecule) or indirect action(the recoilelectron interacts with water to produce an OH, which diffuses to thetarget molecule).

36. Summary of Pertinent ConclusionsAbout two thirds of the biologic damage by x-rays is caused byindirect action.DNA radicals produced by both the direct and indirect action ofradiation are modifiable with sensitizers or protectors.DNA lesions produced by high-LET radiations involve largenumbers of DNA radicals. Chemical sensitizers and protectors areineffective in modifying such lesions.

37. Summary of Pertinent ConclusionsThe physics of the absorption process is over in 10-15 second; the chemistry takes longer because the lifetime of the DNA radicals isabout 10-5 second; the biology takes hours, days, or months for cell killing, years forcarcinogenesis, and generations for heritable effects.

38. Thank you for listening.