1、Radiation Protection and Medical Internal Dosimetry 輻射防護及醫用體內輻射劑量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高雄醫學大學附設中和紀念醫院 核子醫學科核子醫學科 陳陳 毓毓 雯雯 主任主任Pattern of Radiation EffectExposureContaminationUnits of Radiation DoseActivity(A)Bq Radiation exposure C/kgAir keramaAbsorbed dose(D)grayEquivalent dose(H)sievert LET(l
2、inear energy transfer)Effective dose(E)Conversions of UnitsThreshold Doses for Determininstic Effects in The More Radiosensitive Tissues and OrgansBiological Effects of ExposureNonstochastic effectsStochastic effectsProbability of Risk of Fatal Cancer between Age and SexDose in Medical ImagingAnnual
3、 Effective Dose Distribution in The WorldInternal DosimetryThe 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
4、 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 sCumulated activity,As the total number of radioactive disintegrations which occur in the sourve
5、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 A
6、ctivityFlow Chart of MIRD MethologyS-Factors St s =1/mi i i i equilibrium absorbed dose constant i absorbed fraction specific absorbed fraction (Monte Carlo calculations)ExampleCalculated the absorbed dose to the liver of an adult patient who receives 3mCi(111MBq)Tc99m-sulfur colloid for a liver sca
7、n,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 radThe limitations of the MIRD MethodsTabulated doses do not apply to all patients In the MIRD schem
8、a 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.
