Interactions With Matter
Interactions With Matter
Interactions With Matter
Objectives:
1. Historical timeline
2. Define Radiobiology
3. Review of Types of Ionizing Radiation
4. Types of Radiation Interactions with Matter
Historical Perspective
1895 – Röentgen discovered x-rays
1896 – First medical use of x-rays
1897 – First therapeutic use of x-rays
- disappearance of a hairy mole after treatment with x-rays
1898 – Becquerel discovered Radioactivity
1898 – Pierre and Marie Curie isolated Radium
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Inorganic components
HOH
-70%-80%
Salts
-Potassium inside cell
-Sodium outside cell
Organic components
Proteins
-15%
-Monomers vs. polymers
-Amino acids
Carbohydrates
-1%
Nucleic Acid
-RNA and DNA
Lipids
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DNA
-Nitrous bases
-Purines
-Adenine
-Guanine
-Pyrimidines
-Thymine
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-Cytosine
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-S
-G2
Radiobiology defined...
The study of the sequence of events following the absorption of energy from ionizing
radiation, the efforts of the organism to compensate, and the damage to the organism that
may be produced
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Electromagnetic
Particulate
Electromagnetic Radiation
(photons, no mass, no charge, variable energy)
Two forms: x and gamma rays
these do not differ in nature or properties; their designation reflects the way in which
they are produced
X-rays – extranuclearly (means that they are produced in an electrical device that
accelerates electrons to high energy and then stops them abruptly in a target; energy in
motion is converted into x-rays)
Gamma – intranuclearly (means that they are emitted by radioactive isotopes; represent
excess energy given off as unstable nucleus breaks up and decays in its efforts to reach a
stable form)
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significant effect of ionizing radiation is cell killing which is at the root of nearly all normal
tissue and tumor responses noted in patients.
X-rays absorbed in living material
= energy deposited in tissues/cells
This energy is quantized into large individual packets which are big enough to break a
chemical bond and initiate the chain of events that culminates in a biological change
Particulate Radiation
Electrons – small negatively charged particles accelerated to high energy to high speed by
means of electrical device (betatrons, Linear Accelerators)
- widely used for cancer therapy
Protons – positively charged particle and relatively massive
- requires more complex equipment to accelerate (cyclotron)
- used for cancer treatments at specialized facilities
Alpha Particles – nuclei of helium atoms (2 p + 2 n) accelerated in electrical devices similar
to those used for protons
- emitted during decay of heavy radionuclides (i.e. uranium, radium)
- alpha particles produced by radioactive decay of radon are breathed in and irradiates
lining of lung causing 10,000 – 20,000 cases of lung cancer each year
Neutrons – particles with mass similar to proton but carry no electrical charge
- cannot be accelerated in electrical device
- produced if a charged particle (deuteron) is accelerated to high energy and hit
suitable target material
- emitted as a by-product if heavy radioactive atoms undergo fission
- contribute to exposure of passengers and crew of high flying jet liners
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Heavy charged particles – nuclei of elements such as carbon, neon, argon, iron that are
positively charged due to planetary electrons being stripped of them
- to be useful for radiation therapy – they must be accelerated to energies of
thousands of millions of volts so they are only produced at specialized facilities (none
in the U.S. but gaining popularity in Japan and Europe using carbon)
Absorption of X-rays
Absorption of x-rays:
Directly ionizing – charged particles discussed previously
Indirectly ionizing – x and gamma rays (electromagnetic)
The process by which x-ray photons are absorbed depends on energy of the photons
concerned and the chemical composition of the absorbing material
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Absorption of X-rays
Compton – high energies (cobalt-60, LA)
- photon interacts with free electron
- production of large number of fast electron, many ionize other atoms of the absorber,
break vital chemical bonds, and initiate the change of events that express biological
damage
Photoelectric – low energies (diagnostic range)
-photon gives entire energy to bound outer electron which is then ejected and interacts
with tissue as with any other electron
- orbital vacancy within atom typically filled by other orbital electron with excess energy
given off as low energy characteristic x-ray
- low energy radiation with little biological consequences
Absorption of Ionizing Radiation:
The harmful effects of ionizing radiation occur when energy is transferred to atoms or
molecules. The interaction can be direct or indirect action.
-The harmful effects of ionizing radiation occur when energy is transferred to atoms or
molecules. Biological effects of radiation principally result from damage to DNA (critical
target). Ionizing radiation either may interact directly with the critical target or may
indirectly with other parts of atoms and molecules in the cell which will ultimately cause
damage to the critical target.
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Direct action
If any form of radiation is absorbed in biological material, there is a possibility that it will
interact directly with critical targets within cells.
Atoms of the target itself may be ionized or excited, thus initiating the chain of events
that leads to biologic change
Direct action is the dominate mechanism for energies with high linear energy transfer
(LET) such as neutrons or alpha particles.
Absorption of X-rays:
Indirect action
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Ionizing radiation interacts with other atoms or molecules in cell to produce changes
which eventually cause damage to the critical target.
Particularly with HOH, the most common interaction, free radicals may be produced that
are able to diffuse far enough to reach and damage a critical target.
Up to 95% of biological damage caused by ionizing radiation may be due to indirect
action.
The initial interaction occurs with noncritical molecules, such as with radiolysis of water.
The sequence of events begins with photon striking a non-vital molecule (such as
water) and then leads to the creation of a free radical which interacts with other
molecules. Chemical bonds in vital molecules are broken and biological damage occurs.
Something to ponder: Why is indirect action so much more common than direct? What is
the human body primarily composed of, vital molecules or nonvital molecules?
Answer!!
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**Because 80% to 85% of the body is composed of water molecules, these nonvital
molecules are much more likely to be ionized. Although direct action may result in
immediate damage to a vital molecule, there are relatively fewer vital molecules in the body.
**Remember also that the initial ionization is only the first event. The free radicals can go on
to damage many molecules, vital or otherwise.
Free Radicals
A free radical is an atom or molecule that carries an unpaired orbital electron in its outer
shell.
The unpaired electron causes a high degree of chemical instability and reactivity.
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The free radical will try and stabilize itself by either gaining or losing an electron or it
may share an electron with another atom, typically another free radical, that is looking
for another atom.
A free radical may also try and steal an electron from a neighboring molecule to
stabilize itself thus making its victim an electron short so that it now becomes a free
radical. The new free radical may do the same thing thus starting an enormous chain
reaction called a free radical cascade which happens very quickly and can damage
biological tissue.
- Definition: Radiolysis is dissociation of molecules by nuclear radiation
Direct action
Atoms of target may be ionized or excited initiating the chain of events leading to biological
change
= direct action
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This is the dominant process if radiations with high LET (linear energy transfer) such as
neutrons or alpha particles are considered
Indirect Action
Radiation may interact with other atoms or molecules in the cell to produce free radicals that
are able to diffuse far enough to reach and damage the critical targets
= indirect action
Organic molecules contain carbon and can support life. In this illustration the letters RH
symbolize these molecules. As with water molecules, radiation can cause organic
molecules to form free radicals, symbolized as R* and H*. In the absence of oxygen or a
force to attract an electron, these free radicals may reform back into organic molecules.
If oxygen is present, these free radicals can interact with oxygen and form new free
radicals: RO2* and HO2*. When this occurs, the original organic molecules are
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destroyed. The free radicals are able to continue on their path and interact with other
organic molecules. More biologic damage occurs when this happens.
The importance of the interaction of organic molecules and radiation will be discussed in
more detail in the next module in this series.
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The last important factor is time. Normally, we think of time in terms of hours, minutes, or
seconds. Free radicals exist only for about 10-5 second.
However, the period between the damage caused by the free radicals and the manifestation
of the biological effect may be anywhere from hours to years. Cell killing will be manifested
when the cell tries to divide anywhere from hours to days later. It may take generations to
see the manifestation of a genetic mutation.
Absorption of Neutrons
Indirectly ionizing; interact with nuclei of atom of absorbing material
Part of energy of neutron is given to proton as kinetic energy
Elastic collisions of neutrons with heavier elements in tissue make small contribution to the
dose, although energy is deposited at a high LET
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