viii.3. optimization of protection for medical exposures in nuclear medicine 2. dose to patient...
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VIII.3. Optimization of Protection for VIII.3. Optimization of Protection for
Medical Exposures in Nuclear MedicineMedical Exposures in Nuclear Medicine
2. Dose to patient2. Dose to patient
Postgraduate Educational Course in Radiation Protection and the Safety of Radiation Sources
VIII.3.2. Dose to patient 2
CLINICAL DOSIMETRY (BSS)CLINICAL DOSIMETRY (BSS)
II.20. Registrants and licensees shall ensure that the following items be determined and documented:
(d) In diagnosis or treatment with unsealed sources,representative absorbed doses to patients;
VIII.3.2. Dose to patient 3
Conception of absorbed dose in NMConception of absorbed dose in NM
The calculation of the absorbed dose - a tricky The calculation of the absorbed dose - a tricky problem,problem, because of several factors:because of several factors: 1. 1. the distribution of the radionuclide within the the distribution of the radionuclide within the
body and its uptake in certain critical organs body and its uptake in certain critical organs 2.2. inhomogeneous distribution of the nuclide even inhomogeneous distribution of the nuclide even
within the critical organ within the critical organ 3. 3. the biological half-life of the nuclide, which may the biological half-life of the nuclide, which may
vary with patients' ages and may be modified by vary with patients' ages and may be modified by disease or pathological conditions.disease or pathological conditions.
VIII.3.2. Dose to patient 4
Treatment of Hepatocellular Treatment of Hepatocellular Carcinoma with Carcinoma with 131131I-LipiodolI-Lipiodol
CT scan CT scan demonstrating demonstrating
non-non-homogeneous homogeneous distribution of distribution of
Lipiodol. Lipiodol. Tumour can not Tumour can not be treated as a be treated as a homogenous homogenous
spheresphere..
VIII.3.2. Dose to patient 5
Absorbed dose to an organ is determined by:
•Radionuclide•Activity administered•Activity in the organ•Size and shape of the organ•Activity in other organs•Kinetics of radiopharmaceutical•Quality of radiopharmaceutical
VIII.3.2. Dose to patient 6
The MIRD System of The MIRD System of Internal Absorbed Dose Internal Absorbed Dose CalculationCalculation MIRDMIRD - Medical Internal Radiation Dosimetry - Medical Internal Radiation Dosimetry
developed by the Society of Nuclear Medicinedeveloped by the Society of Nuclear Medicine
The organ containing the radionuclide is called The organ containing the radionuclide is called the the sourcesource organ organ
We wish to calculate the absorbed dose to the We wish to calculate the absorbed dose to the targettarget organorgan
The source and target organs may be the sameThe source and target organs may be the same
The amount of radiation from the source The amount of radiation from the source reaching the target must be knownreaching the target must be known
VIII.3.2. Dose to patient 7
Absorbed Dose in the Absorbed Dose in the Target OrganTarget Organ
The absorbed dose will be equal The absorbed dose will be equal to the total amount of energy that to the total amount of energy that is emitted by the source organ is emitted by the source organ XX the fraction of that energy that is the fraction of that energy that is absorbed in the target organabsorbed in the target organ divided divided by by the mass of the target the mass of the target organorgan
VIII.3.2. Dose to patient 8
Determination of the Determination of the Absorbed FractionAbsorbed Fraction
The only method The only method available isavailable is
CALCULATIONCALCULATION
using Monte Carlo using Monte Carlo modellingmodelling
VIII.3.2. Dose to patient 9
What is Monte Carlo What is Monte Carlo Modelling?Modelling? Essentially a ray tracing method, in which the fates of individual Essentially a ray tracing method, in which the fates of individual
particles are determinedparticles are determined
The method is based on randomly sampling a probability The method is based on randomly sampling a probability distribution for each successive interactiondistribution for each successive interaction
Typically, the history of 10 million photons will be modeledTypically, the history of 10 million photons will be modeled
Requires detailed knowledge of the absorption and scattering Requires detailed knowledge of the absorption and scattering coefficients for the specific energies and for the various types of coefficients for the specific energies and for the various types of tissues.tissues.
VIII.3.2. Dose to patient 10
General MIRD EquationGeneral MIRD Equation
D = ÃSD = ÃS (S = nE(S = nE/m)/m)
à is the cumulated activity, à is the cumulated activity, nE is the emitted energy nE is the emitted energy per disintegration, per disintegration, is the absorbed fraction and m is the absorbed fraction and m the mass of the organ.the mass of the organ.
S is dependent on the radionuclide and the geometry. S is dependent on the radionuclide and the geometry. S-values for different radionuclides and source/target S-values for different radionuclides and source/target organs can be found in MIRD publicationsorgans can be found in MIRD publications
VIII.3.2. Dose to patient 11
ICRPICRP
ICRP publications 53, 62 & 80 give the absorbed dose per unit activity administered (mGy/MBq) for different radiopharmaceuticals and different organs as well as the effective dose.
VIII.3.2. Dose to patient 12
Biokinetic modelsBiokinetic models
Extra-cellular
Plasma
Kidneys
Bladder
Injection
Calculate time-activity curves for thedifferent compartments and calculatethe cumulated activity.
VIII.3.2. Dose to patient
Effective doseEffective doseE w HT
TT
Tissue or organ Weighting factorGonads 0.20Bone marrow (red) 0.12Colon 0.12Lung 0.12Stomach 0.12Bladder 0.05Breast 0.05Liver 0.05Oesophagus 0.05Thyroid 0.01Bone surface 0.01Remainder (adrenals, kidney, muscle, 0.05upper large intestine, small intestine,pancreas, spleen, thymus, uterus, brain)
VIII.3.2. Dose to patient
Effective dose - bone scanEffective dose - bone scanOrgan/tissue wT mGy/MBq ProduktGonads 0.20 0.0024 0.000480Bone marrow 0.12 0.0096 0.001152Colon 0.12 0.0038 0.000456Lungs 0.12 0.0013 0.000156Stomach 0.12 0.0012 0.000144Bladder 0.05 0.0500 0.000250Breast 0.05 0.0009 0.000045Liver 0.05 0.0013 0.000065Esophagus 0.05 0.0080 0.000400Thyroid 0.05 0.0010 0.000050Skin 0.01 0.0019 0.000019Bone surface 0.01 0.0630 0.000630Remainder 0.05 0.0019 0.000095
Effective dose (mSv/MBq): 0.0039
VIII.3.2. Dose to patient
Effective dose (mSv)Effective dose (mSv)
0.01
0.1
1
10
cardioangiography thyroid I-131CT pelvis myocard Tl-201large intestineCT abdomen CBF Tc-99murography thyroid I-123lumbar spine bone Tc-99m
thyroid Tc-99mliver Tc-99mlung Tc-99m
chest renography I-131
extremitiesblood volume I-125
dental clearance Cr-51
X-ray Nuclear medicine