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

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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

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Nonlinear Dose-Response

Relationships

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Radiation-induced:

Cancer

Leukemia

Genetic effects

Linear-Nonthreshold

Dose-Response

Relationships

Nonlinear-Threshold

Dose-Response

Relationships

High-dose fluoroscopy–induced

skin effects