VIII.3. Optimization of Protection for Medical Exposures in Nuclear Medicine 2. Dose to patient Postgraduate Educational Course in Radiation Protection and the Safety of Radiation Sources

VIII.3.2. Dose to patient2 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 patient3 Conception of absorbed dose in NM The calculation of the absorbed dose - a tricky problem, because of several factors: n 1. the distribution of the radionuclide within the body and its uptake in certain critical organs n 2. inhomogeneous distribution of the nuclide even within the critical organ n 3. the biological half-life of the nuclide, which may vary with patients' ages and may be modified by disease or pathological conditions.

VIII.3.2. Dose to patient4 Treatment of Hepatocellular Carcinoma with 131 I-Lipiodol CT scan demonstrating non-homogeneous distribution of Lipiodol. Tumour can not be treated as a homogenous sphere.

VIII.3.2. Dose to patient5 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 patient6 The MIRD System of Internal Absorbed Dose Calculation l MIRD - Medical Internal Radiation Dosimetry developed by the Society of Nuclear Medicine l The organ containing the radionuclide is called the source organ l We wish to calculate the absorbed dose to the target organ l The source and target organs may be the same l The amount of radiation from the source reaching the target must be known

VIII.3.2. Dose to patient7 Absorbed Dose in the Target Organ The absorbed dose will be equal to the total amount of energy that is emitted by the source organ X the fraction of that energy that is absorbed in the target organ divided by the mass of the target organ

VIII.3.2. Dose to patient8 Determination of the Absorbed Fraction The only method available is CALCULATION CALCULATION using Monte Carlo modelling

VIII.3.2. Dose to patient9 What is Monte Carlo Modelling? l Essentially a ray tracing method, in which the fates of individual particles are determined l The method is based on randomly sampling a probability distribution for each successive interaction l Typically, the history of 10 million photons will be modeled l Requires detailed knowledge of the absorption and scattering coefficients for the specific energies and for the various types of tissues.

VIII.3.2. Dose to patient10 General MIRD Equation D = ÃS(S = nE  /m) D = ÃS(S = nE  /m) Ã is the cumulated activity, nE is the emitted energy per disintegration,  is the absorbed fraction and m the mass of the organ. S is dependent on the radionuclide and the geometry. S-values for different radionuclides and source/target organs can be found in MIRD publications

VIII.3.2. Dose to patient11 ICRP 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 patient12 Biokinetic models Extra- cellular Plasma Kidneys Bladder Injection Calculate time-activity curves for the different compartments and calculate the cumulated activity.

Effective dose Tissue or organWeighting factor Gonads0.20 Bone marrow (red)0.12 Colon0.12 Lung0.12 Stomach0.12 Bladder0.05 Breast0.05 Liver0.05 Oesophagus0.05 Thyroid0.01 Bone surface0.01 Remainder (adrenals, kidney, muscle,0.05 upper large intestine, small intestine, pancreas, spleen, thymus, uterus, brain)

Effective dose - bone scan Effective dose - bone scan Organ/tissuew T mGy/MBq Produkt Gonads Bone marrow Colon Lungs Stomach Bladder Breast Liver Esophagus Thyroid Skin Bone surface Remainder Effective dose (mSv/MBq):0.0039

Effective dose (mSv) cardioangiography thyroidI-131 CT pelvismyocardTl-201 large intestine CT abdomenCBFTc-99m urographythyroidI-123 lumbar spineboneTc-99m thyroidTc-99m liverTc-99m lungTc-99m chestrenographyI-131 extremities blood volumeI-125 dentalclearanceCr-51 X-ray Nuclear medicine