MAMMALIAN RADIATION BIOLOGY COURSE. Lecture 1 INTERACTION OF RADIATION WITH BIOLOGICAL SYSTEMS. TOPICS COVERED IN THIS LECTURE. Definition of ionizing radiation Types of ionizing radiation and non-ionizing radiation Definition of LET and quality of ionizing radiation
INTERACTION OF RADIATION WITH BIOLOGICAL SYSTEMS
A study of the action of ionizing radiation on living things
LD/50 = 4 Gy
4 Gy = 67 calories
67 calories = 3 ml sip of 60°C coffee
30-100 Trillion Cells at Risk
Radiation is the term given to the energy
transmitted by means of particles or waves.
It can beionizingor non-ionizing
The absorption of energy from radiation in biologic material may lead to excitation or to ionization.
If the radiation has sufficient energy to eject orbital electrons from the atom or molecule, the process is called ionization and that radiation is said to be
The energy per ionizing event – 33 eV – well enough to break
a chemical bond (ex. C=C bond is 4.9 eV).
X-rays and γ-rays do not differ in nature
or in properties, only in the way they are
X-rays (produced extra-nuclearly)
γ-rays (produced intra-nuclearly)
Heavy charged particles
λ - wavelength
ν - frequency
c - velocity
All forms of electromagnetic radiation have the same velocity, but different wavelength, and therefore different frequencies
The Electromagnetic Spectrum
For example, radio wave have a wavelength of 300m; visible light - 5x10-5; X-rays - 1x10-8cm.
as streams of photons, or “packets” of energy
Each energy packet contains an amount of energy equal
to E=hν, wherehis Planck’s constant and ν is the frequency
If a radiation has a long wavelength, it has a small frequency (λν=c),
and so the energy per photon is small. Conversely, radiation with short
wavelength will have a large frequency and hence the energy per
photon is large.
In their biological effects, electromagnetic radiations are considered
to be ionizing if they have a photon energy in excess of 124 eV, which
corresponds to a wavelength shorter than about 10-6 cm.
Excess temperature (0C) = 60 - 37 = 23
Volume of coffee consumed to
equal the energy in the LD/50/60 = 67
= 3 ml = 1 sip
Mass = 70kg
LD/50/60 = 4Gy
Energy absorbed =
70 x 4 = 280 joules
280 = 67 calories
Mass = 70 kg
Height lifted to equal the energy in the LD/50/60 = 280
70 x 0.0981
= 0.4 m (16 inches)
Heavy charged particles
These types of radiation occur in nature and also are used
experimentally, in radiation therapy and diagnostic radiology
Po Pb + He
Scattering: elastic and inelastic; capture; spallation
At high energies
(100 keV-10 MeV)
characteristic of a
cobalt-60 unit or a
used for radiotherapy,
the Compton process
Part of the photon energy is given to the electron as kinetic energy.
The photon proceeds with reduced energy
Neutrons lose their energy by elastic collision with nuclei of similar mass. In soft tissues interaction of a fast neutron with the hydrogen nuclei (protons) is the dominant process of energy transfer.
Part of the energy of the neutron is given to the proton as kinetic energy. Deflected neutron proceeds with reduced energy.
1. Can be absorbed, reflected or transmitted
2. Reflection of laser energy from material surfaces occurs in 2 ways
- Specular - interacts with smooth surfaces and maintains most of the original beam of energy
- Diffuse - interacts with coarse surfaces and disperses the energy in many directions
3. The further laser energy travels within a given material, the more likely the energy will be absorbed within the material
- The laser wavelength and the material with which it interacts determine what percentage of laser energy will be absorbed, reflected or transmitted
is the energy transferred per unit length of the track.
Unit for LET is keV/µm-kiloelectron volt per micrometer
of unit density material.
L=dE/dl, where dE-energy transferred by a charged particle
of specified energy in traversing a distance of dl
Radiation with LET
of 100 keV/µM is the most
efficient in producing
The average separation
between ionizing events
coincides with the diameter
of the DNA double helix
Densely ionizing track of an alpha particular in a cloud chamber
The deposition of energy of different types of radiation
dispersion of energy
high LET (, n, ~)
low LET (, x, ~)
LET = linear energy transfer
The cell is composed of 80% water. The ultimate result of radiation interaction with water molecule is the formation of an ion pair and free radicals. Free radicals have an unpaired electron in their outer shell, a state which confers a high degree of reactivity.
HOH+ + e-
HOH + e- HOH-
H+ + OH•*
OH- + H•
Ion pair (H+, OH-)
Free Radicals (H•, OH•)
X-ray photon fast electron (e) ion radical
free radical chemical changes
Based on the site of these interactions, the action of radiation on the cell can be classified as either direct or indirect.
In indirect action the critical site is damaged by reactive species produced by ionizations elsewhere in the cell, which in turn damage the target
T1/2 in sec Incident X-ray photons
10-15 Fast electrons
10-5 Ion radicals
10-5 Free radicals
Macromolecular changes from breakage of chemical bonds
days - cell killing
generation - mutation
years - carcinogenesis