Common Lab Sources 1 Radioactive Sources 2 Radionuclides in the AZ Particle Lab Gamma Co @ 1uC 241Am, 133Ba, 137Cs, 57Co @ 10 uC 60 60Co, 88Y, 22Na, 64Mg, 203Hg, X-ray 55 Fe 5.90 keV (24.4%) and 6.49 keV (2.86%) Beta 90Sr/90Y @ 50 mCi, 5 mCi, 2mCi, 0.5mCi Alpha 241 Am @ 5 mCi 3 Radionuclides in Medicine Nuclear medicine Diagnostic Permits functional imaging (biochemistry and metabolism versus anatomical structure) >80% of all procedures use 99mTc Radiotherapy Therapeutic Primarily for cancer treatment External beam – teletherapy using 60Co units Internal – brachytherapy using small, encapsulated sources Notes 90% of all radionuclide use in medicine is diagnostic Use of term “radioisotope” is common Will there be a shortage of radionuclides in the future? 4 Radionuclides in Medicine George de Hevesy Nobel in 1943 for use of isotopes as tracers for chemical processes A failed experiment to separate Radium-D (210-lead) from lead (206-lead) The landlady’s leftovers 5 Radionuclides for Diagnosis What are the characteristics of an ideal radionuclide for diagnosis? Half-life? Effective half-life 1/teff = 1/tradioactivity + 1/tbiological Type and energy of radiation? Production and expense? Purity? Target area to non-target ratio? 6 Radionuclides for Diagnosis The ideal gamma energy (for gamma camera use) is between 100 and 250 keV 7 Nuclear Medicine 99mTc is used in ~ 80% of diagnostic procedures Tc pertechnetate (TcO4-) is mixed with an appropriate pharmaceutical (biological construct) for use for 99m Cardiac imaging and function Skeletal and bone marrow imaging Pulmonary perfusion Liver and spleen function Cerebral perfusion Mammography Venous thrombosis Tumor location 8 Technetium – 99m Half-life t1/2=6.02 hrs Decay scheme Which is (are) the medically useful gamma(s)? 9 Technetium – 99m A closer look There is no g1 emission, it IC’s IC competes with g2 IC competes with g3 X-ray and Auger electron emission can also occur 10 Radionuclides for Therapy Brachytherapy Brachys = short Brachytherapy uses encapsulated radioactive sources to deliver a high dose to tissues near the source Provides localized delivery of dose But the tumor must be well localized and small Proposed by Pierre Curie and, independently, Alexander Graham Bell shortly after the discovery of radioactivity Inverse square law determines most of the dosimetric effect 11 Brachytherapy Used to treat a variety of cancers Prostate Gynecological Eye Skin Only ~10% of radiotherapy patients are treated via brachytherapy 12 Brachytherapy Sources Most of the sources used emit gammas Lower gamma energies are preferred for radioprotection 13 Brachytherapy Sources But a few emit betas 90Sr/90Y for eye lesions 90Sr/90Y , 90Y, 32P angioplasty for preventing restenosis after In general, alphas and betas are absorbed by encapsulation to avoid tissue necrosis 14 around the source Nanotargeted Radionuclides Use monoclonal antibodies to carry a radionuclide payload 15 Brachytherapy Sources 226Ra -> 222Rn + a -> … -> 206Pb Although rarely used now, it’s a good reaction to know given its historical significance 16 Brachytherapy Sources 226Ra -> 222Rn + a -> … -> 206Pb Which equilibrium is achieved (t1/2(226Ra) = 1600 years)? 222Rn is a radioactive gas About 50 gamma energies are possible ranging from 0.184 to 2.45 MeV, though on average there are 2.2 gammas emitted for each decay The average energy (filtered by 0.5 mm of Pt) is 0.83 MeV The exposure rate constant (assuming 0.5 mm of Pt) is G = 8.25 R-cm2/hr-mCi 17 Brachytherapy Sources More modern replacements for 137Cs 226Ra are Familiar gamma ray spectrum with E=0.662 MeV t1/2=30 yrs and G=3.26 R-cm2/hr-mCi and 192Ir More complicated gamma ray spectrum with <E> = 0.38 MeV t1/2=73.8 days and G=4.69 R-cm2/hr-mCi 18 Brachytherapy Methods of delivery LDR (0.4-2 Gy/hr) versus HDR (> 12 Gy/hr) Temporary versus permanent Intracavity versus interstitial Also surface, intraluminal, intravascular, intraoperative Seeds, needles, tubes, pellets, wire 19 Brachytherapy 20 Radionuclide Production How are radionuclides made? Primary sources Nuclear reactors 235U fission produced Neutron activated Both produce neutron rich radionuclides Cyclotrons Uses charged particle beams (p, d, t, a) Produces proton rich radionuclides Secondary source Radionuclide generators 21 Nuclear Fission Fission of 236U* yields two fission nuclei plus several fast neutrons 22 Nuclear Reactors Nuclear reactor schematic 23 Fission Production Nuclei such as 99Mo, 131I, and 133 Xe are produced in the fission products using an enriched 235U target (HEU – 90%) Complex chemical processing (digestion or dissolution) and purification separates the 99Mo from chemically similar elements and radiocontaminents The result is a high specific activity (Bq/kg), carrier free nuclide This means there is no stable isotope of the element of interest Some negatives are the potential proliferation of HEU targets and radioactive waste 24 Neutron Activation An alternative use of reactors is to produce radionuclides via neutron activation A X 98 42 X n, g A1 X X Mon, g Mo, Pn, g P 124 54 99 42 31 15 Xen, g Xe I 125 54 32 15 125 53 Two drawbacks of this method are Small activation fraction Chemically similar carrier that cannot be separated 25 Cyclotrons We will cover accelerator physics later in the course 26 Cyclotron Production Cyclotron energies can be a few MeV to a few GeV Laboratory/university or hospital based Beam currents of 40-60 uA Produces Ci-level radioisotopes 14 7 N ( p, a ) C 16 8 O( p, a )137N 15 7 N ( p, n)158O 18 8 Siemens Eclipse 11 6 18 9 O ( p, n) F 27 Cyclotron Production The reactions shown on the previous page Are proton rich -> decay by e+ emission or EC 18F is the most common radionuclide in PET oncology Are important elements of all biological processes hence make excellent tracers 18F is used to label FDG (18F-fluorodeoxyglucose) Useful because malignant tumors show a high uptake of FDG because of their high glucose consumption compared with normal cells Have short lifetimes (O(minutes)) Except t1/2 for 18F = 110 minutes 28 Cyclotron Production 18F in PET/CT 29 Cyclotron Production Alzheimer’s diagnosis 30 Radionuclide Generators Generates a radionuclide by exploiting transient equilibrium Most important application are moly generators 99Mo (67 hours) decaying to 99mTc (6 hours) Sodium pertechnetate (NaTcO4) results which can then be combined with an appropriate pharmaceutical Developed at BNL, a particle and nuclear physics lab Other generators also exist (69Ge to 68Ga, 82Sr to 82Rb, …) 31 Radionuclide Generators Procedure A glass column is filled with aluminum oxide that serves as an adsorbent Ammonia molybdenate attaches to the surface of the resin A sterile saline (the eluant) solution is drawn through the column The chloride ions exchange with the TcO4- but not the MoO4The elute is thus Na+TcO4- (sodium pertechnetate) 32 Radionuclide Generators Technetium cow 33 Radionuclide Generators Generator schematic 34 Radionuclide Generators Generally shipped weekly and milked daily 35 Gamma Camera These images are made using gamma cameras We will cover the details of these (and similar detectors) in upcoming lectures 36 Gamma Camera A schematic of a standard gamma camera 37