Radiation Protection and Medical Internal Dosimetry ?????????????

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Radiation Protection and Medical Internal
Dosimetry ?????????????

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Pattern of Radiation Effect

• Exposure
• Contamination

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Units of Radiation Dose

• Activity (A) Bq
• Radiation exposure C/kg
• Air kerama
• Absorbed dose (D) gray
• Equivalent dose (H) sievert LET
  (linear energy transfer)
• Effective dose (E)

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Conversions of Units
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Threshold Doses for Determininstic Effects in The
More Radiosensitive Tissues and Organs
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Biological Effects of Exposure

• Nonstochastic effects
• Stochastic effects

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Probability of Risk of Fatal Cancer between Age
and Sex
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Dose in Medical Imaging
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Annual Effective Dose Distribution in The World
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Internal Dosimetry

• The method of calculating absorbed dose delivered
  internally has been developed over many years by
  the Medical Internal Radiation Dose (MIRD)
  committee of the American Society of Nuclear
  Medicine.
• The aim of committee was to develop a dosimetry
  system (MIRD schema) for diagnostic nuclear
  medicine. However, the methods have also been
  applied in radionuclide therapy and in internal
  contamination.

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MIRD Schema

• Source Organs vs Target Organs Dt s A
  s St s
• Cumulated activity, As
  the total number of radioactive disintegrations
  which occur in the sourve organ, and depends on
  the activity administered the uptake of ,
  retention by, and excretion from the organ and
  thte physical decay of the radionuclide.
• S-factors
  have been tabulated for a variety of
  radionuclides and for different source/target
  configurations in both standard man and children.

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Cumulated Activity
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Flow Chart of MIRD Methology
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S- Factors

• St s 1/ mi ? ?i ?i
• ?i equilibrium absorbed dose constant
• ?i absorbed fraction
  
• specific absorbed fraction (Monte Carlo
  calculations)

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Example

• Calculated the absorbed dose to the liver of an
  adult patient who receives 3mCi (111MBq)
  Tc99m-sulfur colloid for a liver scan, assuming
  85 liver uptake with no excretion.
• Answer
  Weight of liver
  1700 g (for a standard man)
  A0 in the liver 3000 x 0.85
  2550 u Ci (86.7 MBq) T e 6 hr
  
  ?i ?i
  0.0806
  
• D 1.44 x (2550/1700) x 6 x 0.0806 1.04 rad

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The limitations of the MIRD Methods

• Tabulated doses do not apply to all patients
• In the MIRD schema it is assumed that the
  shape, size and position of the organs are s
  prepresented by the standard, 70kg, hermaphrodite
  human phantom. Disease organs can result in both
  increased or decreased uptake of activiity and
  changes in the residence time compared with
  standard values so these factors sholud also be
  considered when assessing the dose to patients.
  
• The MIRD schema claculates each dose to
• the target organs as an average, without
• permitting the determination of a maximum
• or minimum dose.