The Radiobiology of Radiation
Therapy
Type of Injuries
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Nuclear
Cellular
Nuclear
Cellular
DNA is major target
membrane damage – minor
membrane damage – minor
organelle injury – minor
• Mitochondrial DNA ??
Mechanism
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Two mechanisms of injury
• Direct Ionization of the DNA, ≈ 15%
• Indirect Ionization of the DNA, ≈ 85%
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DNA damaged by free radicals formed in the
micro-environment of the DNA
Water is most important source
Oxygen is important in fixating injury
Sulfhydryl compounds promote repair
Types of DNA Injury
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Base pair injury
Base pair deletion
Base pair cross linkage
Single strand break in backbone
Double strand break in backbone
Gene suppression or activation
Base Pair Injury
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Damage to one of the pairs of
nitrogenous bases in the DNA
sequence.
Easily repaired by cellular repair
mechanisms.
Repair is error free
Base Pair Deletion
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Complete destruction of a pair of the
nitrogenous bases in the sequence
Rapidly repaired by cellular repair
mechanisms
Not necessarily error free repair.
Base Pair Crosslinkage Injury
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Abnormal pairing of the nitrogenous
bases.
May effect conformation of DNA
Repaired efficiently
Single Strand Break
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Result of ionization of the sugarphospate rail of the DNA molecule
Most is easily repaired unless base
pairs are also lost
Repair is rapid and accurate but
some is not repairable.
Double Strand Break
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Breakage of both strands of the DNA
backbone in close proximity to each
other.
Difficult to repair
Repair is quite prone to errors.
High dose and High LET event.
Gene Suppression or Activation
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Radiation injury may result in
upregulation of some genes.
• Tumor Promoter genes
• Tumor Suppressor genes
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Radiation injury may result in down
regulation of the same genes
Down regulation of genes controlling
intracellular repair.
Cell Survival Curves
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Cell survival curve expressed on a
log/linear plot.
Developed through many years of
experimentation
Different curves are derived for
different types of radiation.
Cell survival, neutrons vrs. xrays
Single Hit Killing
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Lethal damage to DNA by single
photon.
Mostly due to double strand breaks
May be due to pro apoptotic gene
activation
Represented by the initial straight
portion of the photon survival curve
Multi-hit Killing
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Lethal injury to the DNA following
multiple hits of the DNA by photon
radiation
Coincident single strand breaks
result in a double strand break
Activation of pro apoptotic genes
Increases with dose
Represented by steep part of curve
Survival Curve Shoulder
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Represents the transition zone
between single and multiple hit
killing
The shoulder is representative of the
repair capability of the cell
population
Wider in slowly dividing cells
Narrower in rapidly dividing cells
Alpha/Beta Ratio
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Really is determined by a dose point
Point on survival curve where single
and multi-hit killing are equal
Larger in cell lines with a wider
repair shoulder.
Alpha/Beta Ratio
LET and Effect on Survival
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LET = Linear Energy Transfer
• Measured in keV/micron
• Characteristic of particulate radiation
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High LET radiation increase killing
per unit energy deposited.
• Results in severe repair deficiencies
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Effectively removes the repair
shoulder
LET and Effect on Survival
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High LET radiation is densely ionizing
Averages >1 ionization event within
the span of a DNA molecule.
High ionization density increases
probability of double strand breaks.
Reaches a maximum effect at about
100 keV/micron.
LET and Effect on Survival
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Photons have an average LET of
about 1.
<1 ionization event within the
diameter of a DNA Molecule.
Single strand breaks predominate
Repair is permitted
LET and Effect on Survival
Cell Cycle and Radiation Injury
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M phase – mitosis very sensitive to
radiation injury
G1 phase – resting phase,
moderately resistant
S phase – DNA synthesis,
moderately resistant to radiation
G2 resting phase – sensitive
G0 non cycling cells – moderate
resistance
Cell Cycle and Radiation Injury
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Mitosis
• Chromosomes are condensed
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DNA is closely packed – bigger target
• Repair mechanisms are shut down
• Very compressed time scale = 1 hr.
• Any DNA injury is fixed in place
• Cell may loose large segments of DNA
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Fragments excluded from nucleus
Cell Cycle and Radiation Injury
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S phase
• Phase of DNA synthesis
• Most radiation resistant phase
• Cellular repair mechanisms are active
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Increases repair of radiation damage
• Lasts about 5 hours.
Cell Cycle and Radiation Injury
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G1
• Functional part of cell cycle
• Resistance varies with part of phase
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Goes down as cell nears the G1-S interface
Point in cell cycle where apoptosis occurs
• Cell death at this point is referred to as
interphase death
• Longest part of cycle.
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Lasts hours to years
Cell Cycle and Radiation Injury
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G2
• Short rest phase before M
• Quite radiation sensitive
• Short time allows little for injury repair
• Radiation injury incurred in S-phase
may be repaired
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May result in a mitotic delay in G2
• Apoptosis-like death may also occur
The Four R’s
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Repair
Reassortment
Reoxygenation
Repopulation
Repair
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Rapid repair of injury
Initiated within seconds of injury
Complete by 6 hours after injury
Can be modified by environmental
conditions
• Presence or absence of oxygen or free
radical scavengers.
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Responsible for shoulder of survival
curve
Reassortment
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When cells killed in sensitive phases
it leave a gap in the cell population
for those phases.
Within two cycles cells from less
sensitive parts of cycle replace them
Some non-cycling cells may be
recruited into the cycling pool.
Reoxygenation
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Most tumors larger than 1 cm have
some hypoxic cells in them
• Some tumor types have larger %
• May be transient or chronic
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Radiation preferentially kills
oxygenated cells (O2 fixation of injury)
Major contributor to tumor radiation
resistance.
Reoxygenation
Reoxygenation
Repopulation
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Following killing of cells in a
population by any means there is
either replacement or repopulation of
the cells killed
Usually there is days to weeks delay
before this begins
Tissues with large clonogenic
populations are able to do this better
Repopulation
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Tends to be a low dose phenomenon
Usually is most important in rapidly
cycling cell population.
• This includes tumors
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Rapid repopulation may reduce level
of repair
Tissue Level Radiation Effects
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All mammalian cells equally sensitive
in cycling populations in cell culture
However, in tissue the rate of cell
replacement is variable
Some cell populations turn over
every 3-5 days and some never do.
• Cell growth fractions and cell death
fractions should be in balance.
Tissue Effects
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Radiation response at tissue level is
tied to cell death
• Cell death is mostly tied to cell
reproduction
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Apoptosis
• Radiation induction of apoptosis pathways
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Mitotic linked death
• Reproductive failure due to missing DNA
• Long cell cycle times blunt response
Tissue Effects
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Long cell cycle times promote repair
and slow repopulation
Short cell cycle times promote
repopulation and blunt repair
Large non-cycling populations blunt
radiation response
Dose required to inhibit function is
much higher than that for
reproductive inhibition or failure.
Tissue Effects
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At the tissue level the ultimate
survival of the tissue depends on:
• The number of cycling cells
• The ability of the tissue to repair the
injury.
• The ability of the tissue to repopulate
the tissue with the original cell type.
Tissue effects
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Repopulation is most important at
low doses;
Early responding tissues tend to
have more repopulation
Late responding tissues tend to have
limited repopulation capability
• Therefore sensitive to larger doses of
radiation.
Tissue Effects
Radiation Delivery
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Treatment with a number smaller
doses improves normal tissue
response and increases total dose
that can be given to a tumor
• Reduces hypoxia
• Promotes repopulation in late
responding tisues
• Promote reassortment
• Promotes repair of DNA injury
Fractionation
Fractionation
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Optimal dose is that which is just
about midway through the repair
shoulder.
Usually approximately equal to the
Do dose
Must wait at least 6 hours for repair
to be complete.