Radiation units

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RADIATION UNITS

Moderator: Dr. Teerthraj VermaPresenter: Dr. Mansi Barthwal

Junior ResidentDepartment of Radiation Oncology

King George’s Medical UniversityLucknow

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• 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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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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RADIATION UNITS

UNITS OF RADIOACTIVITY• Curie (Ci)• Becquerel (Bq)UNITS OF RADIATION DOSES• Unit of Exposure:

Roentgen (R)• Unit of Absorption

Physical dose: Rad/GrayBiological dose: rem/sievert

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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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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⁻²FLUENCE RATE OR FLUX DENSITY(ø):• Fluence per unit time(dt).

ø=dф dt

• Its unit is m⁻²s⁻¹

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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⁻²ENERGY 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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PHOTON BEAM ATTENUATION*

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• 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=-μNdxWhere μ is the constant of proportionality, called attenuation coefficient. *• In terms of Intensity(I)

dI= -μdxIIf thickness x is expressed as a length, then μ is called the linear attenuation coefficient.

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• 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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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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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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EXPOSURE

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• 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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• 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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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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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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ABSORBED DOSE AND KERMA• The ratio of absorbed dose to collision kerma at a

point is β

Range of electron

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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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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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• 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/kg1Gy = 1 J/kg = 100rad

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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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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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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: rem1 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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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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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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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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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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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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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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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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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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