1. Radiobiology for the Radiologist, Hall, 7th ed
Chapter 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 Röntgen,
“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 γ-rays,
two forms of electromagnetic radiation.
• Do not differ in nature or in properties.
– X-rays are produced extranuclearly.
– γ-rays are produced intranuclearly.
• Everything that is stated about x-rays in this chapter applies equally
well to γ-rays.

8. 1. Electromagnetic Radiations
• X-rays as waves of electrical and magnetic energy.
• Velocity = c
• Wavelength = λ
• Frequency(the number of waves passing a fixed point per second) =
ν
λ
λν = 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 = ℎν

10. 1. Electromagnetic Radiations
• X-rays as streams of photons, or “packets” of energy.
– Each energy packet equal to hν.
– λÅ = 12.4/E(keV)
• for example, x-rays with wavelengths of 0.1Å 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 (λ = 10-6 cm).

12. 2. Particulate Radiations
• Electrons, protons, α-particles, neutrons, negative π-
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
• α-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 γ-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 = ℎν – 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 α-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'-atom
by 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 of
Neutrons, 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 γ- rays,
not for neutrons, α-particles, or heavier ions.

35. Summary of Pertinent Conclusions
X- and γ-rays are indirectly ionizing; the first step in their absorption is the
production of fast recoil electrons.
Neutrons are also indirectly ionizing; the first step in their absorption is the
production of fast recoil protons, α-particles, and heavier nuclear
fragments.
Biologic effects of x-rays may be caused by direct action(the recoil
electron directly ionizes the target molecule) or indirect action(the recoil
electron interacts with water to produce an OH·, which diffuses to the
target molecule).

36. Summary of Pertinent Conclusions
About two thirds of the biologic damage by x-rays is caused by
indirect action.
DNA radicals produced by both the direct and indirect action of
radiation are modifiable with sensitizers or protectors.
DNA lesions produced by high-LET radiations involve large
numbers of DNA radicals. Chemical sensitizers and protectors are
ineffective in modifying such lesions.

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

38. • Thank you for listening.