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Copyright © 2013 by Mosby, Inc., an affiliate of Elsevier Inc. All rights reserved.
Fundamental Principles of
Radiobiology on Living Systems
Mod 4 – Part #2 – Bushong
Chapt. 3010-526-197
Rhodes
2
Law of Bergonie and Tribondeau 1906 French scientists
Stem cells are radiosensitive; mature cells are radioresistant.
Younger tissues and organs are radiosensitive.
Tissues with high metabolic activity are radiosensitive.
High proliferation rate for cells and high growth rate for tissues result in increased radiosensitivity.
Blood Cells - .25 Gy ( 25 Rad) can cause measurable
Hematologic depression
Immature Blood Cell Depletion
Muscle Tissue – Specialized - radioresistent
Nerve Tissue – Highly specialized - radioresistent
4
Physical Factors Affecting
Radiosensitivity
Linear Energy Transfer ( LET )
Relative Biologic Effectiveness ( RBE )
Protraction & Fractionation
A measure of the rate at which energy is transferred to tissue
Utilizes radiation Quality and weighting factors
Expressed in units of kiloelectronvolt per micrometer of soft tissue ( keV/µm)
All radiations do not produce the same biologic effect
Formula to compare amount of different types of radiation to consistent injury
RBE = Dose of standard radiation necessary to produce a given effect
Dose of test radiation necessary to produce the same effect
A biologic reaction in produced by 2 Gy of a test radiation. It takes 10 Gy of 250 kvp x-rays to produce the same reaction. What is the RBE of the test radiation?
Fractionated doses have the same dose delivered over a long period of time.
Protracted doses are delivered continuously at lower rates
Methods cause less damage due to time for intacellularrepair
9
Biological Factors Affecting
Radiosensitivity
Oxygen Effect
Age
Recovery
Chemical Agents
Hormesis
Tissue is more sensitive when oxygenated
Aerobic versus Anoxic
Oxygen Effect
Oxygen Enhancement Ratio (OER) =Dose necessary under anoxic conditions to produce effect
Dose necessary under aerobic conditions to produce effect
OER highest for low LET radiations
Humans most sensitive before birth
Decreases until maturity
Most resistant during adulthood
Old age increases sensitivity
Cells can recovery from radiation damage
Interphase death occurs when cell dies before replicating
Recovery is a combination of repair and repopulation from adjacent cells
Chemical modification of cell response
Must be present at time of exposure
Radiosensitizers – Most double the sensitivity
Radioprotectors - Most double protection
Some radiation studies suggest that radiation can be good for a species.
Live longer
16
Radiation Exposure
High Dose
Deterministic Response -nonstochastic
Late ResponseEarly Response
Low Dose
Stochastic Response -Probabilistic
17
Dose-Response Relationships
Deterministic
Radiation-induced skin
burns
Stochastic
Cancer
Leukemia
Genetic effects
Mathematical relationship between radiation doses and observed response
Graph that charts dose on X- axis
Response measured on Y – axis
Curves or straight lines show relationships
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Linear Dose-Response
20
Nonlinear Dose-Response
Relationships
21
Radiation-induced:
Cancer
Leukemia
Genetic effects
Linear-Nonthreshold
Dose-Response
Relationships
Nonlinear-Threshold
Dose-Response
Relationships
High-dose fluoroscopy–induced
skin effects
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