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.