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