Introduction to Health Physics Chapter 6 Radiation Dosimetry

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Introduction to Health PhysicsChapter
6Radiation Dosimetry
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UNITS

• During the early days of radiological experience,
  there was no precise unit of radiation dose that
  was suitable either for radiation protection or
  for radiation therapy
• Furthermore, since the fraction of the energy in
  a radiation field that is absorbed by the body is
  energy-dependent, it is necessary to distinguish
  between radiation exposure and radiation absorbed
  dose

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Absorbed Dose

• Gray
• Radiation damage depends on the absorption of
  energy from the radiation and is approximately
  proportional to the concentration of absorbed
  energy in tissue
• 1 Gy 1 J/kg
• Rad
• 1 rad 100 ergs/g
• 1 Gy 100 rads

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Exposure

• The exposure unit is a measure of the photon flux
  and is related to the amount of energy
  transferred from the X-ray field to a unit mass
  of air. One exposure unit is defined as that
  quantity of X- or gamma radiation that produces,
  in air
• 1 X unit 1 C/kg air

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The Roentgen

• The roentgen is an unit of exposure ( X ). The
  ICRU defines 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 ( or - ) liberated by photons in air
  of mass dm are completely stopped in air.
• X dQ / dm
• The SI unit is C/kg but the special unit is
  roentgen ( R )
• 1R 2.58 10-4 C/kg

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Measurement The Free Air Chamber

• Charged Particle Equilibrium (CPE ) Electron
  produced outside the collection region, which
  enter the ion-collecting region, is equal to the
  electron produced inside the collection region ,
  which deposit their energy outside the region.

• Example 6.2

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Radiation Absorbed Dose

• Exposure photon beam, in air, Elt3MeV
• Absorbed dose for all types of ionizing
  radiation
• Absorbed dose is a measure of the biologically
  significant effects produced by ionizing
  radiation
• Absorbed dose dE/dm
• dE is the mean energy imparted by ionizing
  radiation to material of dm
• SI unit gray (Gy) 1Gy 1 J/kg
• ( 1 rad100ergs/g10-2J/kg, 1cGy1rad )

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Relationship Between Kerma, Exposure, and
Absorbed Dose

• Kerma ( K ) Kinetic energy released in the
  medium.
• K dEtr / dm
• dEtr is the sum of the initial kinetic energies
  of all the charged particles liberated by
  uncharged particles ( photons) in a material of
  mass dm
• The unit for kerma is the same as for dose, that
  is, J/kg. The name of its SI unit is gray (Gy)
  

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Relationship Between Kerma, Exposure, and
Absorbed Dose

• Kerma ( K ) Kcol and Krad are the collision and
  the radiation parts of kerma
• K Kcol Krad
• 
  ( J / m2 ) ( m2 / kg )
• the photon energy fluence, ?
• averaged mass energy absorption coefficient, men
  / r

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Relationship Between Kerma, Exposure, and
Absorbed Dose

• Exposure and Kerma
• Exposure is the ionization equivalent of the
  collision kerma in air
• (Kcol)air X ( w/e ) , X dQ/dm
• w/e 33.97 J/C

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Relationship Between Kerma, Exposure, and
Absorbed Dose

• Absorbed Dose and Kerma

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Relationship Between Kerma, Exposure, and
Absorbed Dose

• Absorbed Dose and Kerma
• Suppose D1 is the dose at a point in some
  material in a photon beam and another material is
  substituted of a thickness of at least one
  maximum electron range in all directions from the
  point, then D2 , the dose in the second material,
  is related to D1 by

D1
D2
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D1
D2
maximum electron range
maximum electron range
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Calculation of Absorbed Dose from Exposure

• Absorbed Dose to Air
• In the presence of charged particle equilibrium
  (CPE), dose at a point in any medium is equal to
  the collision part of kerma.
• Dair ( Kcol )air X ( w/e )
• Dair(rad) 0.876 ( rad/R) X (R)

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Calculation of Absorbed Dose from Exposure

• Absorbed Dose to Any Medium
• Under CPE
• Dmed / Dair (men/r)med / (men/r )air A
• A ?med / ?air
• Dmed(rad) fmed X (R) A
• fmed roentgen-to-rad conversion factor

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Calculation of Absorbed Dose from Exposure

• Absorbed Dose to Any Medium

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Calculation of Absorbed Dose from Exposure

• Dose calculation with Ion Chamber In Air
• For low-energy radiations, chamber wall are thick
  enough to provide CPE.
• For high-energy radiation, Co-60, build-up cap
  chamber wall to provide CPE.

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Relationship Between Kerma, Exposure, and
Absorbed Dose

• Example 6.4
• Consider a gamma-ray beam of quantum energy 0.3
  MeV. If the photon flux is 1000 quanta/cm2/s, and
  the air temperature is 20?, what is the exposure
  rate at a point in this beam and what is the
  absorbed dose rate for soft tissue at this point?

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The Bragg-Gray Cavity Theory

• Limitations when calculate absorbed dose from
  exposure
• Photon only
• In air only
• Photon energy lt3MeV
• The Bragg-Gray cavity theory, on the other hand,
  may be used without such restrictions to
  calculate dose directly from ion chamber
  measurements in a medium

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The Bragg-Gray Cavity Theory

• Bragg-Gray theory
• The ionization produced in a gas-filled cavity
  placed in a medium is related to the energy
  absorbed in the surrounding medium.
• When the cavity is sufficiently small, electron
  fluence does not change.
• Dmed / Dgas ( S / r )med / ( S / r )gas
• (S / r)med / (S / r)gas mass stopping power
  ratio for the electron crossing the cavity

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The Bragg-Gray Cavity Theory

• Bragg-Gray theory
• Dmed / Dgas ( S / r )med / ( S / r )gas
• Jgas the ionization charge of one sign produced
  per unit mass of the cavity gas

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The Bragg-Gray Cavity Theory

• The Spencer-Attix formulation of the Bragg-Gray
  cavity theory
• F(E) is the distribution of electron fluence in
  energy
• L/r is the restricted mass collision stopping
  power with ? as the cutoff energy

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INTERNALLY DEPOSITED RADIOISOTOPES

• Corpuscular Radiation
• The calculation of the absorbed dose from
  internally deposited radioisotopes
• specific effective energy (SEE)
• The energy absorbed per unit mass per
  transformation
• For practical health physics purposes,
  "infinitely large" may be approximated by a
  tissue mass whose dimensions exceed the range of
  the radiation from the distributed isotope. For
  the case of alpha and most beta radiation, this
  condition is easily met

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INTERNALLY DEPOSITED RADIOISOTOPES

• Example 6.11
• Calculate the daily dose rate to a testis that
  weighs 18g and has 6660 Bq of 35S uniformly
  distributed throughout the organ

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INTERNALLY DEPOSITED RADIOISOTOPES

• Effective Half-Life
• The total dose absorbed during any given time
  interval after the deposition of the isotope in
  the organ may be calculated by integrating the
  dose rate over the required time interval
• In situ radioactive decay of the isotope
• Biological elimination of the isotope

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INTERNALLY DEPOSITED RADIOISOTOPES

• Total Dose Dose Commitment

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INTERNALLY DEPOSITED RADIOISOTOPES

• Total Dose Dose Commitment
• For practical purposes, an "infinitely long time"
  corresponds to about six effective half-lives

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INTERNALLY DEPOSITED RADIOISOTOPES

• Total Dose Dose Commitment
• Compartment theory
• In many cases, an organ or tissue behaves as if
  the radioisotope were stored in more than one
  compartment
• Each compartment follows first order kinetics and
  is emptied at its own clearance rate

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INTERNALLY DEPOSITED RADIOISOTOPES

• Total Dose Dose Commitment
• Compartment theory
• Since the activity in each compartment
  contributes to the dose to that organ or tissue

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INTERNALLY DEPOSITED RADIOISOTOPES

• Gamma Emitters
• cannot simply calculate the absorbed dose by
  assuming the organ to be infinitely large because
  gammas, being penetrating radiations, may travel
  great distances within tissue and leave the
  tissue without interacting

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INTERNALLY DEPOSITED RADIOISOTOPES

• Gamma Emitters

• C is the concentration of the isotope
• G is the specific gamma-ray emission
• m is the linear energy absorption coefficient

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INTERNALLY DEPOSITED RADIOISOTOPES

• Gamma Emitters

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INTERNALLY DEPOSITED RADIOISOTOPES

• Gamma Emitters
• geometry factor, g

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INTERNALLY DEPOSITED RADIOISOTOPES

• Gamma Emitters
• Average geometry factor, g

• For a cylinder

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• Gamma Emitters
• Example 6.12
• A spherical tank, capacity 1 m3 and radius 0.62
  m, is filled with aqueous 137Cs waste containing
  a total activity of 37,000 MBq (1Ci). What is the
  dose rate at the tank surface if we neglect
  absorption by the tank wall?

Surface0.5center
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• MIRD Method
• To account for the partial absorption of
  gamma-ray energy in organs and tissues, the
  Medical Internal Radiation Dose Committee of the
  Society of Nuclear Medicine (MIRD) developed a
  formal system for calculating the dose to a
  "target" organ or tissue (T) from a "source"
  organ (S) containing a uniformly distributed
  radioisotope

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• MIRD Method
• based on the concept of absorbed fraction, that
  is, the fraction of the energy radiated by the
  source organ which is absorbed by the target
  organ. S and T may be either the same organ or
  two different organs bearing any of the possible
  relationships to each other
• These absorbed fractions are calculated by the
  application of Monte Carlo methods to the
  interactions and fate of photons or electrons
  following their emission from the deposited
  radionuclide

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• MIRD Method

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• MIRD Method
• Monte Carlo methods
• events such as the interaction of photons with
  matter are governed by probabilistic rather than
  deterministic laws
• individual simulated photons (or other
  corpuscular radiation) are "followed" in a
  computer from one interaction to the next
• we know the energy of the emitted radiation, its
  starting point, and its initial direction. The
  probability of each possible type of interaction
  within the organ and the energy transferred
  during each interaction are also known

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• MIRD Method
• Monte Carlo methods
• A situation is simulated by starting with a very
  large number of such nuclear transformations,
  following the history of each particle as it
  traverses the target tissue, and summing the
  total amount of energy that the particles
  dissipate within the target tissue

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• MIRD Method
• Monte Carlo methods
• absorbed fraction usually is less than 1
• For non-penetrating radiation, the absorbed
  fraction usually is either 1 or 0, depending on
  whether the source and target organs are the same
  or different

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• MIRD Method
• Example 6.13
• calculations of the dose rate to a 0.6-kg sphere
  made of tissue-equivalent material in which 1 MBq
  of 131I is uniformly distributed
• The total energy absorbed from the 131I is simply
  the sum of the emitted beta-ray energy plus the
  fraction of the emitted gamma-ray energy that is
  absorbed by the sphere

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• MIRD Method
• Example 6.13

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• MIRD Method
• Example 6.13

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• MIRD Method
• Example 6.13, return to the MIRD method
• Let us consider two organs in the body
• The rate of energy emission by the radionuclide
  in the source at any time that is carried by the
  ith particle is given by

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• MIRD Method
• Example 6.13, return to the MIRD method

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• MIRD Method
• Furthermore, since the radioactivity is usually
  widespread within the body, a target organ may be
  irradiated by several different source organs.
  The dose to the target, therefore is
• where rk represents the target organ and rh
  represents the source organ

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• NEUTRONS
• The absorbed dose from a beam of neutrons may be
  computed by considering the energy absorbed by
  each of the tissue elements that react with the
  neutrons
• For fast neutrons up to about 20 MeV, the main
  mechanism of energy transfer is elastic collision
• Thermal neutrons may be captured and initiate
  nuclear reactions

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• NEUTRONS
• For fast neutrons
• For isotropic scattering, the average fraction of
  the neutron energy transferred in an elastic
  collision with a nucleus of atomic mass number M
  is

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• NEUTRONS
• For thermal neutrons
• Exp. 14N( n, p )14C reaction
• Exp. 1H( n, g )2H reaction

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PROBLEMS

• 6.1, 6.2, 6.3, 6.4, 6.6, 6.10, 6.11,