Tissue Radiation Biology

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Tissue Radiation Biology
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Response to irradiation at the tissue level

• Tied to cellular division kinetics
• In general cells have the same sensitivity to
  ionizing radiation as far as nuclear injury is
  concerned.
• The DNA in all mammalian cells has about the same
  sensitivity to radiation injury.,

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Response to irradiation at the tissue level

• Difference in response become apparent at the
  tissue (organ) level.
• These differences in radiation sensitivity are
  due to the rate of replication inherent in the
  critical cells in that

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"Law" of Bergonie' and Tribondeau

• Radiation has a more rapid (is more effective)
  effective against cell that are actively
  dividing, are undifferentiated and have a large
  dividing future.

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Cell differentiation

• Undifferentiated cells are precursor or stem
  cells and have less specialized functions. Their
  major role is to reproduce to replace themselves
  and to provide cells which mature into more
  differentiated cells.

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Modified by Ancel and Vitemberger

• The appearance of radiation damage is dependent
  on two factors 1. The biologic stress on the
  cell and 2. the conditions to which the cell is
  exposed pre and post irradiation
• The most important biologic stress is division
  therefore rapidly dividing cells express damage
  earlier and slowly dividing cells later.

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Cell differentiation

• The more specialized a cells function is, the
  more differentiated it is. (examples are the
  major organ cells, muscle and neurons
• Highly differentiated cell usually have less
  reproductive activity than undifferentiated
  cells. (examples of undifferentiated cells are
  bone marrow cells, intestinal crypt cells and
  basal cells of the skin.

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Cell differentiation

• Undifferentiated cells are precursor or stem
  cells and have less specialized functions. Their
  major role is to reproduce to replace themselves
  and to provide cells which mature into more
  differentiated cells.
• Undifferentiated cells generally are actively
  dividing and have a long dividing future.

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Rubin and Casarett

• classification of cellular populations
• based on reproductive kinetics
• These classifications cells is an attempt to
  explain the difference in observed cellular and
  tissue radiosensitivity based on the reproductive
  and functional characteristics of various cell
  lines.

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Vegetative Intermitotic Cells.(VIM)

• Undifferentiated rapidly dividing cells which
  generally have a quite short life cycle. Examples
  are erythroblasts, intestinal crypt cells and
  basal cells of the skin.
• Essentially continuously repopulated throughout
  life.

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Differentiating Intermitotic Cells (DIM)

• Actively mitotic cells with some level of
  differentiation. Spermatogonia are a prime
  example as well as midlevel cells in
  differentiating cell lines.
• Have substantial reproductive capability but will
  eventually stop dividing or mature into a
  differentiate cell line

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Multipotential Connective Tissue Cells

• Cells which divide at irregular intervals often
  in response to a need. Relatively long cell life
  cycle.
• Major examples are fibroblasts although recently
  more examples of such cells have been identified
  in a number of tissues

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Reverting Postmitotic Cells (RPM)

• does not normally undergo division but can do so
  if called upon by the body to replace a lost cell
  population. These are generally long lived
  cells.
• Mature liver cells, pulmonary cells and kidney
  cells make are examples of this type of cell.

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Fixed Postmitotic Cells. (FPM)

• These cells do not and cannot divide.
• They are highly differentiated and are highly
  specialized in there morphology and function.
• May be very long lived or relatively short lived
  but replaced by differentiating cells below them
  in the cell maturation lines.
• Examples are Neurons, muscle cells and RBCs

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Perceived Radiation Sensitivity

• VIM cells are the most sensitive cells to
  radiation and FPM cells are most resistant. The
  others are of intermediate sensitive in the order
  presented.
• However, this perception is a product of the
  longer cell cycle time in more highly
  differentiated cell lines

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Michalowski Classification

• A more modern type of classification which
  essentially says the same thing in another way.

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Michalowski Classification

• Stem cells continuously divide and reproduce to
  give rise to both new stem cells and cells that
  eventually give rise to mature functional cells.
• Maturing cells arising from stem cells and
  through progressive division eventually
  differentiate into an end-stage mature functional
  cell.
• Mature adult functional cells that do not divide

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(H-type)

• There are many cell types that progress from the
  stem cell through the mature cell with
  nonreversible steps along the way. These cell
  lines are said to be hierarchical (H-type)
  populations.
• They include bone marrow, intestinal epithelium,
  epidermis and many others.

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F-type populations

• There are other cell lines in which the adult
  cells can under certain circumstance be induced
  to undergo division and reproduce another adult
  cell. These cell are said to be flexible tissue
  (F-type populations).
• Examples include liver parenchymal cells,
  thyroid cells and pneumocytes as well as others.

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Michalowski Classification

• These two types represent extremes and there are
  many tissues which exhibit characteristics of
  both types where mature cells are able to divide
  a limited number of times.
• The rapidity of response to and hence the
  sensitivity to radiation at the tissue level is
  dependent on the length of the life cycle and the
  reproductive potential of the critical cell line
  within that tissue.

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Critical Cells"

• All tissues contain multiple cell types contained
  in either the stromal compartment or the
  parenchymal compartment.
• A cell in either compartment may be the critical
  cell.

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Critical Cells"

• the endothelial cells lining the blood vessels
  were thought for many years to be the critical
  cells in tissues however
• "critical cells" have been identified in many
  tissues.

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The time required for the tissue to respond to
radiation injury can be predicted on the basis of
the cell cycle kinetics of these critical cells.
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Biologic Factors moderating Cell injury by
irradiation.

• Cell Cycle.
• Intracellular repair
• Hypoxia

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Cell Cycle.

• The point that a cell is in the cell cycle has a
  marked influence on its response and survival of
  irradiation.
• G1 G0 are relatively insensitive to radiation
  injury.
• S phase is generally considered to be the most
  resistant to radiation injury.

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Cycle Phase Influence on Sensitivity
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Intracellular repair

• The shoulder on the cell survival curve indicates
  that there is some degree of repair by cells of
  radiation injury.
• Amount of repair differs between cell lines
• However the rate of repair is the same

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Intracellular Repair
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Intracellular repair
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Intracellular repair

• Studies have shown that although repair can be an
  ongoing process, the vast majority of the repair
  is finished by 6 hours post irradiation.
• Once repair is complete the remaining cell
  population will respond to subsequent dose of
  radiation as though the original irradiation had
  not occurred

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Hypoxia

• Oxygen is a potent preventer of repair
• Hypoxia markedly improves the ability of the
  cells to repair radiation injury
• However it is quite rare for a normal somatic
  cell to be hypoxic.

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Measurement or radiation injury at the tissue
level

• Assay systems are needed to construct survival
  and injury curves for irradiation at the tissue
  level.
• Such assays must be quantifiable
• The effect measured must increase with dose

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Types of Assays

• Clonogenic (related to reproductive potential of
  stem cells in the tissue target cell population
• Specific tissue functional capability
• Lethality - death of the organism from radiation
  of that tissue

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Clonogenic assays

• May be performed in vivo or in vitro
• In an in vitro assay cells are harvested from
  tissue irradiated in living tissue and the cells
  are grown out in cell culture and the number of
  colonies growing out is compared to that for a
  control
• In vivo assays are performed by evaluation of
  cellular reproductive activity in the living
  animal

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In Vitro Assays

• Cells harvested from culture and plated out
  many, many flasks or dishes
• Dishes are irradiated at different levels
• The number of colonies are counted after a
  specific time.
• of colonies compared to control sample
• Survival curves generated

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In vivo Assays

• Two types
• In Situ Assays
• Transplantation Assays

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In Situ Assays

• The tissue or organ is irradiated in the whole
  animal. At a given time after irradiation the
  organism (animal or plant) is sacrificed and the
  organ of interest is evaluated for cell survival
  of the cell of interest.
• Classic example is the intestinal crypt cell
  studies

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In Situ Assays

• Another example is irradiation of testes and then
  assaying the testicle for surviving spermatogonia
  in the tubules of the testicle.

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In Situ Assays

• Classically these assays have been used to
  evaluate the radiation effects in acutely
  responding (rapidly dividing) cell lines such as
  the intestinal villi, the testes and the skin.
• Recently these types of assays have been extended
  to evaluation (slowly dividing) cell lines.

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In situ Assays

• These assays have shown that the Do for slowly
  dividing cells in this assay is about 1.5 Gy or
  about the same as for the rapidly responding
  tissues. The difference in time required for the
  cell killing to occur is a manifestation of the
  slow turnover rate of the cells.

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In Situ Assays

• Tissue is irradiated in vivo and returned to
  subject.
• After a period of time the number of viable cell
  groups in irradiated area is measured
• Generally done in mice as large numbers are
  required.
• Intestinal and gonadal epithelium are the classic
  tissues studied.

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In Situ Assays

• Studies of RPM and FPM tissues and cell lines
  requires much longer experiment
• May require use of larger more expensive and long
  lived animals
• These studies are very expensive to do

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Transplantation Assays

• Often used to study tumor sensitivity
• Usually done in immune compromised animals.
• Done to mimic metastatic disease or to remove
  immune system effects in live animal

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Transplantation Assays

• Tumor or test tissue is irradiated while still in
  donor animal.
• Irradiated tissue is then removed and cells
  suspended in solution.
• Cells then injected into recipient animal
• After a period of growth, the animals are
  sacrificed and the number of tissue colonies or
  size of colony is measured.

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Functional Assays

• Measure organ functional capacity
• Most organs have clinical functional reserve
• Tests measure complete functional capacity
• Done in a live animal with in situ organs
• Do not require sacrifice of the animal
• Multiple dose levels can be studied
• Measure effect at sub clinical levels.
• Heart, lungs, liver, kidneys applicable

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Functional Assays

• Measure effects of regional irradiation
• Very important in radiation therapy
• Helps predict effects of irradiation plan
• Also used to study effects of ingested
  radionuclides either medical or accidental
• Useful for studies on effect modifiers such as
  chemotherapy.

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Lethality Assays

• Measures clinical effects
• Measures doses required to cause death.
• Whole body irradiation
• Regional body irradiation (brain, heart, liver,
  etc.
• Generally expressed in terms of death in a
  given time. i.e. LD30/90
• 30 of subjects die by 90 days post irradiation