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Radiation Protection and Medical Internal Dosimetry 輻射防護及醫用體內輻射劑量
高雄醫學大學附設中和紀念醫院 核子醫學科 陳 毓 雯 主任
Pattern of Radiation Effect
ExposureContamination
Units of Radiation DoseActivity (A) Bq Radiation exposure C/kgAir keramaAbsorbed dose (D) grayEquivalent dose (H)
sievert LET (linear energy transfer)
Effective dose (E)
Conversions of Units
Threshold Doses for Determininstic Effects in The More Radiosensitive Tissues and Organs
Biological Effects of Exposure
Nonstochastic effectsStochastic effects
Probability of Risk of Fatal Cancer between Age and Sex
Dose in Medical Imaging
Annual Effective Dose Distribution in The World
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.
MIRD SchemaSource 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.
Cumulated Activity
Flow Chart of MIRD Methology
S- Factors
St s = 1/ mi Δi i
Δi equilibrium absorbed dose
constant
i absorbed fraction
specific absorbed fraction
(Monte Carlo calculations)
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
Δi i = 0.0806
D = 1.44 x (2550/1700) x 6 x 0.0806 = 1.04 rad
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 maximumor minimum dose.