A consise presentation on radiation units

1. RADIATION UNITS
Moderator: Dr. Teerthraj Verma
Presenter: Dr. Mansi Barthwal
Junior Resident
Department of Radiation Oncology
King George’s Medical University
Lucknow
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2. • Quantities, when used for the quantitative
description of physical phenomenon or objects, are
generally called physical quantities.
• A unit is a selected reference sample of a quantity
with which other quantities of the same kind are
compared.
• Every quantity may be expressed as the product of a
numerical value and a unit.
• A quantity remains unchanged when the unit in
which it is expressed changes, its numerical value is
modified accordingly.
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3. NEED OF MEASURING SYSTEM
• To quantify biological effects in Radiotherapy and
Radiodiagnosis.
• To quantify dose sufficient to produce response.
• To quantify damage.
• To improve clinical practice based on the
observations.
• Direct measurement is impractical.
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4. RADIATION UNITS
UNITS OF RADIOACTIVITY
• Curie (Ci)
• Becquerel (Bq)
UNITS OF RADIATION DOSES
• Unit of Exposure:
Roentgen (R)
• Unit of Absorption
Physical dose: Rad/Gray
Biological dose: rem/sievert
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5. RADIOACTIVITY
• Number of disintegrations per second.
• Conventional unit: Curie(Ci)
• S.I. unit: Becquerel (Bq)
• One curie is defined as activity of material which has
3.7х10¹ᴼ disintegrations per second.
• A source has an activity of 1 becquerel if its
disintegration rate is one per second.
• Therefore, 1 Ci = 3.7х10¹ᴼ Bq.
1 Bq = 1dps = 2.70х10⁻¹¹ Ci *
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6. PHOTON BEAM DESCRIPTION
• Many terms are used to describe a photon beam, these
include:
FLUENCE (ф):
• Quotient dN by da, where dN is the number of photons
that enter an imaginary sphere of cross-sectional area da.
Ф=dN
da
• Its unit is m⁻²
FLUENCE RATE OR FLUX DENSITY(ø):
• Fluence per unit time(dt).
ø=dф
dt
• Its unit is m⁻²s⁻¹
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7. ENERGY FLUNCE(Ѱ):
• Quotient of dEfl by da; where dEfl is the sum of the
energies of all the photons that enter a sphere of cross-
sectional area da.*
Ѱ=dEfl
da
• Its unit is Jm⁻²
ENERGY FLUENCE RATE/ ENERGY FLUX DENSITY OR
INTENSITY (ψ):
• Enregy fluence per unit time.
Ψ=dѰ
dt
• Its unit is Wm⁻¹
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8. PHOTON BEAM ATTENUATION*
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9. • Reduction in the number of photons (dN) is
proportional to the number of incident photons (N)
and to the thickness of the absorber (dx) i.e.
dN=-μNdx
Where μ is the constant of proportionality, called
attenuation coefficient. *
• In terms of Intensity(I)
dI= -μdxI
If thickness x is expressed as a length, then μ is called
the linear attenuation coefficient.
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10. • Attenuation coefficient depends on
-energy of photon
-nature of material (thickness, density)
• MASS ATTENUATION COEFFICIENT: Dividing μ by
density (ρ), the resulting coefficient (μ/ρ) will be
independent of density; is known as mass
attenuation coefficient.
It has a unit of cm²/g
• If the absorber thickness is expressed in units of
electrons/cm² and atoms/cm², the corresponding
coefficients are electronic attenuation coefficient (ₑμ)
and atomic attenuation coefficient (ₐμ), respectively.
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11. ENERGY TRANSFER COEFFICIENT(μtr)
• In a photon beam traversing a material, the fraction
of photon energy transferred into kinetic energy of
charged particles per unit thickness of absorber.
μtr= Ētrμ
hν
Where Ētr is the average energy transferred into kinetic
energy of charged particles per interaction.
• Mass energy transfer coefficient is given by μtr/ρ.
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12. ENERGY ABSORPTION COEFFICIENT(μₑᵑ)*
• Product of energy transfer coefficient and (1-g)
where g is the fraction of the energy of secondary
charged particles that is lost to bremsstrahlung in the
material.*
μₑᵑ=μtr(1-g)
• Mass energy absorption coefficient is given by μₑᵑ/ρ
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13. EXPOSURE
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14. • It is a measure of ionization produced in air by photons.*
• The ICRU defines exposure (X) as the quotient of dQ by dm
where dQ is the absolute value of the total charge of the
ions of one sign produced in air when all the electrons
(negatrons and positrons) liberated by photons in air of
mass dm are completely stopped in air.
X=dQ
dm
• SI unit of exposure is coulomb per kilogram (C/kg)
• Special unit of exposure is roentgen (R).
• 1esu=3.333х10⁻¹⁰C and 1cmᶾ of air at STP weighs
1.293х10⁻⁶kg.
1R=2.58х10⁻⁴ C/kg air
Therefore 1R*= 1esu/cmᶾ
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15. • Measures ionization in air only.
• Cannot be used for photon energies >3MeV.
• Roentgen is unit of X or Gamma ray only and cannot
be used for beta ray or neutron.
• For these RAD was introduced in 1956.
• In 1962, Roentgen was accepted as unit of exposure
and rad as unit of absorbed dose.
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16. KERMA(K)
• Kinetic Energy Released in Medium.
• Defined as “the quotient of dEtr by dm, where dEtr is the sum of
the initial kinetic energy of all the charged ionizing particles
(electrons and positrons) liberated by uncharged particles (photons)
in a material of mass dm.”
K = dEtr
dm
• Unit is J/kg, SI unit is Gray(Gy) and special unit is rad.
• Kerma at a point is directly proportional to the photon energy
fluence Ψ
K = Ψ(μtr/ρ)
where μtr/ρ is the mass energy transfer coefficient for the medium
averaged over the energy fluence spectrum of photons.
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17. COLLISION AND RADIATIVE KERMA
• Major part of initial kinetic energy of electrons in low
atomic number materials is expended by inelastic
collision (ionization and excitation) with atomic
electrons.
• Only a small part is expended in the radiative
collision with atomic nuclei (bremsstrahlung).
K = Kᶜᴼᴸ+ Kᴿᴬᴰ
EXPOSURE AND KERMA
• Exposure is the ionization equivalent of the collision
kerma in air.
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18. ABSORBED DOSE AND KERMA
• The ratio of absorbed dose to collision kerma at a
point is β
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Range of electron

19. DOSE
• In the early days, amount of radiation absorbed was
estimated by reddening of human skin.
• Skin erythema dose (SED) was defined as that
amount of X or γ radiation that just produce
reddening of the human skin.
• Its drawback were, skin erythema depends on skin
type, quality of radiation, extent of skin exposed,
dose fractionation.*
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20. ABSORBED DOSE
• Quantity used to measure the amount of ionization
radiation.
• It is a measure of the biologically significant effects
produced by ionization radiation.
• Defined as ‘energy absorbed per unit mass’.*
• Unit is joules per kilogram or Gray (Gy).*
• 1 Gy= 100 rad
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21. • Conventional unit was radiation absorbed dose (rad),
defined as energy absorption of 100 erg/g.
• Absorbed dose is the quotient dĒ/dm, where dĒ is
the mean energy imparted by ionizing radiation to
material of mass dm.
1rad= 100 ergs/g= 10⁻²J/kg
1Gy = 1 J/kg = 100rad
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22. CONVERSION OF ROENTGEN
TO RAD
• Absorbed energy/gram = energy in the beam х
mass absorption coefficient
= E х mass absorption coefficient
• After some calculation we get
Absorbed energy/gram = fR rads
(factor f is the conversion factor which varies with the
material and with radiation energy.)
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23. CONVERSION FACTOR (f)
WATER MUSCLE BONE
100KV 0.87 0.91 4.23
200KV 0.94 0.94 1.46
1MeV 0.96 0.95 0.91
4MeV 0.96 0.955 0.92
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24. EQUIVALENT DOSE
• Product of absorbed dose averaged over the tissue
or organ and Wʀ selected for type and energy of
radiation involved.
Equivalent dose = absorbed dose х Wʀ
• Conventional Unit: rem
1 rem is dose of any ionizing radiation which will
produce the same biological effect as 1 rad of Co60
• SI unit: sievert (Sv)*
1 Sv = 100 rem
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25. COMMITTED EQUIVALENT DOSE
• Irradiation from internally deposited radionuclides, the
total absorbed dose is distributed over time as well as
to different tissues in the body.
• To take into account the varying time distribution of
dose delivery, the ICRP defined committed equivalent
dose as the integral over 50 years* of the equivalent
dose in a given tissue after intake of a radionuclide.
COLLECTIVE EQUIVALENT DOSE
• Product of the average equivalent dose to a population
and the number of persons exposed.
• Appropriate for application to exposure of a group or
population.
• Unit is person-sievert.
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26. RADIATION WEIGHTING
FACTOR(Wʀ)
• Probability of a stochastic effect depends on dose,
type and energy of radiation.*
• Wʀ is a dimensionless multiplier to place biologic
effects (risks) from exposure to different types of
radiation on a common scale.
• Chosen by ICRP as representative of RBE applicable
to low dose and low dose rates.
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27. RADIATION TYPE Wʀ
Photons 1
Electrons 1
Protons and charged particles 2
α- particles , fission fragments, heavy ions 20
Neutrons Continous curve as a function of neutron
energy(max 20)
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28. TISSUE WEIGHTING
FACTOR(Wт)
• Equivalent doses to various tissue differ substantially
and different tissues vary in their sensitivity to
radiation induced stochastic effects.
• Wт represents the relative contribution of each
tissue or organ to the total detriment resulting from
uniform irradiation of the whole body.
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29. Remainder tissues are adrenals, extrathoracic tissue, gall bladder, heart, kidney,
lymphatic nodes, muscle, oral mucosa, pancreas, prostate, small intestine, spleen,
thymus, uterus/cervix.
ORGAN/TISSUE
NUMBER OF
TISSUES
Wᴛ
TOTAL
CONTRIBUTION
Lung, stomach,
colon, bone
marrow, breast, and
remainder
6 0.12 0.72
Gonads 1 0.08 0.08
Thyroid, esophagus,
bladder, and liver
4 0.04 0.16
Bone surface, skin,
brain, and salivary
glands
4 0.01 0.04
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30. EFFECTIVE DOSE
• The sum of all of the weighted equivalent doses in all the tissues or
organs irradiated is called the effective dose.
Effective Dose= Σ absorbed dose х Wʀ х Wт
• It is a non-measurable quantity.
COMMITTED EFFECTIVE DOSE
• Committed equivalent doses to individual organ or tissues
multiplied by the appropriate Wт.
COLLECTIVE EFFECTIVE DOSE
• Product of the average effective dose to a population and the
number of person exposed.
• Unit is person-sievert.
COLLECTIVE COMMITTED EFFECTIVE DOSE
• A population ingesting or inhaling radionuclides that deposit their
dose over a prolong period, the integral of the effective dose over
the entire population out to a period of 50 years is called the
collective committed effective dose
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31. QUANTITY DEFINITION UNIT
Absorbed dose Energy per unit mass Gray
For individuals
Equivalent dose Absorbed dose х radiation weighting
factor
Sievert
Effective dose Sum of equivalent dose to organs or
tissues exposed, each multiplied by the
appropriate tissue weighting factor
Sievert
Committed equivalent
dose
Equivalent dose integrated over 50 years Sievert
Committed effective dose Effective dose integrated over 50 years Sievert
For populations
Collective effective dose Product of the average effective dose
and the number of individual exposed
Person-sievert
Collective committed
effective dose
Integration of the collective dose over 50
years
Person-sievert
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32. 32

33. 33

34. Quantities that are measured in sievert are intended to represent the
stochastic health risk, which for radiation dose assessment is defined as the
probability of cancer induction and genetic damage.
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35. THANK YOU
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