1

Radiation and Radioactivity

• What Is Radiation Biology?– What type of Radiation?– Directly Ionizing vs. Indirectly Ionizing

– Definition: Radiation Biology is the study of the interaction of ionizing radiation with living systems.

What Do We Need To Know• Radiation Sources – Man-made & natural• Mechanisms of Energy Transfer &

Absorption• Radiation Chemistry• Radiation Effects on Cells - Cellular

Radiation Biology• Radiation Effects on Tissues & Organisms• Late Radiation Effects

• Exposure - the Roentgen (R)

• Dose - rad & gray (G)• Relative Biological Effectiveness - RBE

• Dose Equivalent - rem & sievert

Radiation Quantities & Units

mQX

∆∆

=

D =∆ED∆m

H = D(Q)(N)

RBEx =DR

DX

2

Radioactivity• Emissions from radioactive elements

– Alpha (α) particles: – Beta (β) particles: – Gamma (g) & X-rays photons:– NeutronsUnits of radioactivity

• Units of Radioactivity– curie (Ci) = 3.7 X 1010 dps– becquerel (Bq) = 1dps

• Radioactive decayteNN λ−= 0

teAA λ−= 0

+222 He

+− e ,eνλν hE ,C ==

22

1 mvKE =}

Sources of Ionizing Radiation

• Natural background Radiation– Primordial radionuclides

• 238U, 232Th, 235U production of Radon 222Rn• 40K & 87Rb

– Cosmogenic Radionuclides• 14C, 3H, 7Be, 22Na

– Man made sources– Exposure from Radiation in the environment

Mechanisms of Energy Transfer

• Electromagnetic Radiation - γ & X-rays– Energy Absorption– Energy Transfer Processes

• Photoelectric effect• Compton Scattering• Pair Production

3

Energy Transfer: Photoelectric Effect

Energy Transfer: Compton Scatter

Ee = hν – h ν’

Energy Transfer: Pair Production

4

Mechanisms of Energy Transfer

• Energy Transfer by Neutrons– Nuclear collisions generate High LET

Radiations• Charged Particle Interactions

– Heavy Charged Particles - P+, α++, cosmic– High Kinetic Energy Electrons - e-

Mechanisms of Energy TransferMass=M

e , m=m 0-

F

Fy

x

Distance = r

Distance = b

∆ E ( b ) =z 2 ro

2 mo c 4 Mb 2 E

v

Mechanisms of Energy Transfer

N0

X

N'

N'=NOe−µx

hν

Photons

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Radiation Chemistry

• Composition of Living Protoplasm• Energy Deposition Events

– High Kinetic Energy e-

– Spurs; Blobs; Short Tracks

Radiation Chemistry of H2O: Initial Events

• Excitation and Ionization– e- e-’ + H2O* H• + OH•

– e- e-’ + e-aq + H2O+ OH• + H+

• Initial Reactive Products– OH• H• e-

aq

Radical Interactions

• Recombination• Hydrogen Extraction• Addition Reactions• Oxygen & Fixation of Damage

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Radiation Chemistry of H2O

Direct and Indirect Action

Restoration of Damaged Molecules

• Recombination• Restitution

– Cystein– Glutathione

• Repair

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DNA As The Primary Target

• Consequences of DNA Damage• Evidence for DNA as the target• Types of DNA damage• Consequences of Chain Scission in DNA

– Single strand breaks (SSB)– Double strand breaks (DSB)

Radiation Effects on Cells

• Clonogenic Survival and the Survival Curve• Cell Survival Models

– Target (Lea, 1955)• Multitarget Single Hit (MTSH) Function

– D0, Dq, n

– Molecular (Chadwick & Leenhouts, 1981)• Linear-Quadratic Function

∞α & β parameters

Clonogenic Survival

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Multi-Target Single Hit Model

The Molecular Model10.0

S = e−(αD+βD 2 )

2 4 6 8 10 12

0.1

0.01

0.001

1.0

5.0

Surv

ivin

g Fr

actio

n (S

)

Dose (Gy)

Modification of Cell Survival

• Cell Age Effects• Damage Repair - SLD vs. PLD• Oxygen Effects - OER• Radiation Protectors and Sensitizers

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Damage RepairSub-Lethal Damage

RepairPotentially-Lethal Damage Repair

Oxygen Effects (OER)

Radiobiology of Tissues

• Classification of Tissues• Normal Tissue Radiosensitivity• The Acute Radiation Syndrome• Effects on the Developing Embryo and

Fetus

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Classification of Tissues

Acute Radiation Syndrome

Effects on Embryo & Fetus

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Late Radiation Effects

• Nonstochastic vs. Stochastic Effects• Carcinogenesis

– Data sources– Multistep process and Clonal theory– Risk Estimation

• Mutagenesis

Experimental Tumor Incidence