A ray of black on cells: A short review on radiation biology
Introduction
When X rays interact with human body there are various changes that take place in the body.
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Total Radiation
How are tissues affected by x-rays
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Effects in atoms and molecules
Excitation: If the energy of excitation is too large, it may produce a break in the molecular bond and disrupt the molecule
Ionization: An ionized becomes more chemically reactive as it attempts to find an electron to find the missing one
Breaking of molecular bonds: If enough molecules are damaged, there is a loss of cell function, disruption of internal organelles, or even death of the cell. 1, 2, 3, 4
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Chemical effects of radiation
Radiolysis of water
Ionization of water photoelectron/photon - ion pair
Phaton+H2O→e- +H2O+Photoelectron e-+H2O→2e- +H2O+
2. Formation of Hydroxyl free radical
OH0+OH0→H2O2H0+02→HO20
Time scale of radiation damage
Latent period
The time interval between irradiation and development of the observed biological effect is known as latent period.5
Accumulative effects of radiation
Each succeeding radiation exposure is assumed to add a small increment to the radiation damage.
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Dosimetry
Equivalent dose(HT): The equivalent dose is used to compare the biological effects of different types of radiation to a tissue or organ. HT =Σ WR *DT
Effective dose(E : Is used to estimate the risk in humans E= Σ WT *HT
Radioactivity(A : Describes the decay rate of a sample of radioactive material.
Classification of radiobiological effects
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Deterministic effect
Response is proportional to the dose
Cell death
Threshold
Eg. effect on oral cavity
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Stochasatic effect
Probability of the response the dose
All or none
No threshold
Eg. Radiation induced cancer
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Radiation affects life in two ways
Direct effect
Indirect effect
Direct effect
Photon or 2* electron hit bio-molecule-ionization
1. Free radical production
RH+X-ray→R0+H++e
2. Free radical fates
R0→x+Y
• Cross linking
R0+S0→RS
Indirect effect
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Displaced electron reacts with water molecule
Negative water molecule
e-+H2O→H2O-
Unstable and dissociate into
Hydroxyl ion and
Hydrogen free radical H2O-→OH-+H0
Effects on biological molecules
Effects on nucleic acids
Effects on proteins
Effects on cell kinetics
Effects on nucleic ACIDS
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Breakage of 1 or both DNA strands.
Cross linking of DNA strands with the helix, or to other DNA strands
Change or loss of base.
Disruption of Hydrogen bonds between DNA strands
Effect on proteins
Affects the secondary and tertiary structure
Affects side chains, hydrogen and sulfide bonds
Inter and intra molecular connection
Amplification of effect by influencing enzymes
Less significant than DNA damage
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Radiation effect on cell kinetics
Effect on high mitotic index population
Decrease in tissue size
Cell cycle delay
Cell death
Bystander effect
Cells that are damaged by radiation, release into immediate environment their products causing cell aberrations, mutations, carcinogenesis
Survival curve
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They are used to study the response of replicating cells exposed in culture to radiation
MODIFYING FACTORS
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Radiation sensitivity and cell type (Bergoni and Tribondeau 1960
Different cells react to radiation in different ways
Most sensitive:
Casarett’s classification of tissue radiosensitivity
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Effects on organs
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Total vs Whole body exposure
Refers to area of body exposed
Extensive radiation injury > damage that occurs in blood-forming tissues
Linear energy transfer
The rate at which energy is transferred from ionizing radiation to soft tissue, expressed in terms of kiloelectron volts per micrometer (keV/μm) of track length in soft tissue. The LET of diagnostic x-rays is about 3 keV/μm, 6, 7
Mosby's Medical Dictionary, 8th edition. © 2009, Elsevier
eg: alpha particles
eg; X- ray
Conflicts of Interest
All contributing authors declare no conflicts of interest.
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