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.