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Physics of Novel Radiation Modalities:
Radionuclides
James S. Welsh
Stritch School of Medicine
Loyola University Chicago
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Disclosure
• Member of the Advisory Committee on the Medical Uses of Isotopes (ACMUI) for the United States Nuclear Regulatory Commission (NRC)
• Board of directors:
– Coqui Radioisotopes
– Colossal Fossils
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Learning Objectives
• Understand the basic physics of alpha, beta, gamma and other types of radioactivity
• Gain some familiarity with the various sealed and unsealed radionuclides commonly used in radiation oncology
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Types of radioactivity• Alpha• Beta
– Beta minus– beta plus (positron emission)– electron capture
• Gamma– Isomeric transitions– Internal conversion– Internal pair production
• Cluster radioactivity• Spontaneous fission
– Binary or ternary
• Rare types:– Proton radioactivity– b+ delayed proton emission– b- delayed neutron emission– b+ delayed deuteron or triton emission– Beta delayed fission
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Fun with Isotopes
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• Supposedly unaffected by temperature, pressure, chemical environment
• First declared by Rutherford, Chadwick and Ellis
Radioactive decay supposedly follows a mathematically precise exponential function
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Generally true but…
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…well-known exceptions do exist
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• Electron Capture (e.g. 7Be, 109In, 110Sn)
– If chemical environment make K-shell electrons less accessible, decay rate might be altered
…well-known exceptions do exist
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• Electron Capture (e.g. 7Be, 109In, 110Sn)
– If chemical environment make K-shell electrons less accessible, decay rate might be altered
• Isomeric Transitions
– 99mTc: observable half-life changes due to chemical environment
– T1/2 difference ~0.3% when in Tc2S7 vs NaTcO4
(sodium pertechnetate) in physiological saline
…well-known exceptions do exist
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Is it stable???
• Z > 83 (bismuth)????
– If so, the isotope is unstable
– Every (natural) element from 84 (Po) upwards is radioactive
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Is it stable???
• Z > 83 (bismuth)????
– If so, the isotope is unstable
– Every (natural) element from 84 (Po) upwards is radioactive
– Even Bi-209 might be unstable…
– with an α-emission half-life of 1.9×1019 years
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Is it stable???
• Recall:• Z = number of protons• N = number of neutrons• A = number of protons + neutrons (i.e. total number
of nucleons)
• Are both Z and N even?– If so, the isotope is probably stable (e.g. C-12, O-16)
• Are both Z and N odd?– If so, the isotope is probably unstable (e.g. F-18)• Oddness of both Z and N tends to lower the nuclear
binding energy
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Odds of being stable
Protons Neutrons Number of Stable Nuclides Stability
Odd Odd 4 least
Odd Even 50 less
Even Odd 57 more
Even Even 168 most
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Is it stable???
• Is there a “magic number” of nucleons?
– If so, the isotope is stable
– Results in complete nuclear shells
– High average binding energy per nucleon
• Protons: 2, 8, 20, 28, 50, 82, 114
• Neutrons: 2, 8, 20, 28, 50, 82, 126, 184
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Double the magic
• Nuclei with both N and Z each being one of the magic numbers are “double magic”
• Only 10 of ~2500 nuclides• Unusually stable against decay (note: this does NOT
mean they are absolutely stable!)• Some double magic isotopes include
– helium-4– oxygen-16– calcium-40– nickel-48– nickel-78– lead-208
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Is it stable???
• What is the N:Z ratio?
• Where is the isotope in relationship to the “zone of stability”?
• In other words - Is it in the zone?
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Regarding the zone• As Z increases, A must increase disproportionately for
stability
– Number of neutrons needed increases as the number of protons increases
• Fe-56 is the most stable isotope (lowest mass per nucleon)
– Below Fe-56 fusion can generate energy
– Above Fe-56 fission can generate energy
• No natural elements with Z > 83 (bismuth) are stable
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Regarding the zone• Stable light nuclides contain about equal protons and
neutrons
• Stable heavy elements contain up to 1.6x more neutrons than protons
• Nuclides above (to the left of) the band of stability are neutron-rich
• Nuclides below (to the right of) the band are neutron deficient
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Neutron-rich nuclides• To the left of the zone: Need more protons
– Want to rid the excess n and produce more p
• Below Z=83, neutron-rich radioisotopes decay via beta minus emission – (i.e. conversion of a neutron into a proton)
• Above Z=83, neutron-rich nuclei also decay via alpha emission
• Note: alpha decay actually increases the n:p ratio– e.g. 238U92
234Th90 + 4He2
– 146n and 92p (n:p = 1.587) vs 144n and 90p (n:p = 1.6)
– Daughters tend to be more n-rich than the parents
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Some more definitions
Examples
Isotopes Same Z, different A 131I53 125I53
Isotones Same N, different A and/or Z 39Ar1840K19
Isobars Same A, different Z 228Ra88228Th90
Isodiaphers Excess mass (N-Z) is the same 235U92231Th90
Isomers Same Z, same A (different energy) 99mTc 99Tc
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• On this particular diagram style:
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• Isotopes on horizontal line
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• Isobars on NE line (beta decay)
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• Alpha decay on vertical line
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Alpha decay
• Ejection of a Helium nucleus
• AXz A-4Yz-2 + 4He2
• Requires:
• Mx > My + MHe
– 210Poz 206Pb + 4He2
– (209.9829u) (205.9745u) + (4.0026u)
• 209.9829u > 209.9771u Therefore a allowed
• Cu-64 cannot alpha decay
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Polonium-210
• T1/2 = 138 days
• 5.3 MeV
• 166,500 TBq/kg (4500 Ci/g)
• Extremely toxic: 1 mg can kill an average adult
– ~250,000x more toxic than HCN by weight
• Used to kill Russian dissident Alexander Litvinenko in 2006
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Americium-241• A trans-uranium actinide
• Ordinary household smoke detectors contain ~0.29 mg of americium dioxide
• Am-241 alpha decays to Np-237– T1/2 = 432.2 years
• a collide with O and N molecules in the air
• Generates ions in the ionization chamber– Ions produce an electric current between electrodes
• Ions are neutralized upon contact with smoke– Decreasing the electric current
– Activates the detector's alarm
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Plutonium-238• Half-life of 87.7 years
• Powerful alpha emitter– Does not emit significant g
• Radioisotope Thermoelectric Generators (RTGs) – Converts heat into electricity via Seebeck effect
– 1g Pu-238 generates approximately 0.5W
– Voyager 1 and 2, Cassini–Huygens, New Horizons and the Mars Science Laboratory
• 250 plutonium-powered cardiac pacemakers made:– 22 were still in service more than 25 years later
– No battery-powered pacemaker could achieve that!
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Radium-226
• T1/2 = 1600 years• Alpha decay to Rn-222• 6th Member of the Uranium Series - ultimately
ending in Pb-206• 78 g rays from Ra-226 and decay products • Energy ranging from 0.184 MeV - 2.45 MeV (these
photons are what were clinically useful)– Average 0.83 MeV
• HVL 14 mm Pb• 0.5 mm Pt encapsulation for beta particle filtering
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Primordial radionuclide decay series
• Thorium series (n)
• Neptunium series (4n+1)
• Uranium series (4n+2)
• Actinium series (4n+3)
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• Thorium series
• 4n series
• "Decay Chain Thorium" by http://commons.wikimedia.org/wiki/User:BatesIsBack -http://commons.wikimedia.org/wiki/File:Decay_Chain_of_Thorium.svg. Licensed under CC BY-SA 3.0 via Wikimedia Commons
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• Neptunium series
• 4n+1 series
• Extinct
• "Decay Chain(4n+1, Neptunium Series)" by BatesIsBack -http://commons.wikimedia.org/wiki/File:Decay_chain(4n%2B1,Neptunium_series).PNG. Licensed under CC BY 3.0 via Wikimedia Commons
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• Uranium series
• 4n+2 series
• "Decay chain(4n+2, Uranium series)" by User:Tosaka -File:Decay chain(4n+2, Uranium series).PNG. Licensed under CC BY 3.0 via Wikimedia Commons -
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• Actinium series
• 4n+3 series
• "Decay Chain of Actinium" by Edgar Bonet - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons -
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Radium Basics
• One gram of radium-226 undergoes 3.7 × 1010
disintegrations per second
• Thirty-three isotopes of radium
– All radioactive
• Half-lives (generally) short:
– less than a few weeks
– with the exceptions of radium-226 (1,600 years) and radium-228 (5.8 years)
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Biological effects• Radium dermatitis:
• Only 2 years after its discovery, A. Henri Becquerel developed a skin ulcer after carrying an ampule in his pocket for six hours
• Marie Curie developed a skin ulcer after a few days following 10 hrs of direct contact with a tiny sample
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“The Radium Craze”• 1903 - numerous commercially available products
became available– Cosmos Bag for arthritis– Liquid Sunshine– Radiathor
• The sad case of Eben Byers ended this era upon his death in 1932– He consumed an estimated 1400 bottles of
Radiathor– This Wall Street Journal line said it all:– "The Radium Water Worked Fine…
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“The Radium Craze”• 1903 - numerous commercially available products
became available– Cosmos Bag for arthritis– Liquid Sunshine– Radiathor
• The sad case of Eben Byers ended this era upon his death in 1932– He consumed an estimated 1400 bottles of
Radiathor– This Wall Street Journal line said it all:– "The Radium Water Worked Fine until his jaw came off”
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The Radium Girls
• U.S. Radium Corporation
• Watch dial luminous paint containing 70 mg/g of paint
• Contained RaBr and ZnS (which glows upon alpha irradiation)
• Of 800 employees from 1917 to 1924, 48 developed radiation sickness (including mandibular necrosis) and 18 died (including cases of osteosarcoma)
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The Great Radium Scandal. Roger Macklis. Scientific American 1993
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So why is there possibly any interest in Radium today???
• Radium-223 is the isotope of interest presently
• Part of the actinium series (4n + 3 series)
• Radiologically well-suited for radiopharmaceuticals
• 11.4-day half-life
• 5.99 MeV alpha emission
• First FDA-approved unsealed source alpha-emitting radiopharmaceutical
• Some compelling clinical data has emerged recently
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Radium-223 Decay Chain
• Of the total decay energy
– 95.3% emitted as a particles
– 3.6% emitted as b particles
– 1.1% emitted as g or x-rays
• Easily measured on standard dose calibrators
223Ra11.43 d
219Rn3.96 s
α
α
α
α
β−
β−
β−
α
215Po1.78 ms
α
211Pb36.1 m
207TI4.77 m
211Bi2.17 m
211Po516 ms
207Pbstable
(0.27%)
(99.73%)
Henriksen et al. Cancer Res. 2002;62:3120-3125. 46
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Nilsson et al Clin Cancer Res 2005
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Radium-223
• Bone-seeking like the beta emitters Sr-89 and Sm-153 EDTMP
• But Ra-223 is a high-LET alpha emitter
• α-particles cause double-strand DNA– Limited penetration of α particles (~ 2-10 cell diameters)
• In principle: – potentially more effective at killing tumor cells
– less myelosuppressive due to range <100 mm of alpha particles
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Spontaneous fission
• Although possible, not prevalent in nature
• U-238 decays via spontaneous fission 2 million x slower than its already slow alpha decay (4.5 Ga vs10 Pa)
• For artificial radionuclides with Z>90, this does occur
• Typically with emission of one or more neutrons (up to 10)
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An Isotopic Source of Neutrons: Cf-252
• Alpha decay (97%) and Fission (3%):
– 252Cf 248Cm + 4a (96.9%)
– 252Cf fission + 1n (3.1%)
• Average of 3.7 neutrons per fission
• Neutron energy range of 0 to 13 MeV
• Mean value of 2.1 MeV and most probable energy 0.7 MeV
• T1/2 = 2.64 years
• Average photon energy 0.8 MeV
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Cluster Decay and Ternary Fission
• AKA Cluster Radioactivity or heavy particle radioactivity
• Nucleus emits a small "cluster” of neutrons and protons
– Larger than alpha particles
– Smaller than a normal binary fission fragment
• 223Ra → 209Pb + 14C
• Ternary fission into three fragments can also produce products in this size range
– Although 3rd fission product most often is a He nucleus
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Beta decay
• Parent and daughter are isobars
• Electron emission: “Beta minus”
• Positron emission: “Beta plus”
• Electron capture
• Can be considered “inverse beta decay”
• But inverse beta decay also refers to:
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Beta minus decay• Converts a neutron into a proton (Z increases)• An electron is emitted from the nucleus• A SPECTRUM of energies• Neutron becomes a proton plus an electron plus an (electron)
antineutrino
• Recall that neutrinos (n) exist in 3 flavors:– ne, nm, nt
• Equations must balance:– Mass (baryons)– Charge– Matter/anti-matter– Lepton number– Energy– Momentum
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Welsh JS. Am J Clin Oncol 2007;30: 437–439)
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At a more granular level…
• A neutron (composed of 2 down quarks and 1 up quark) is converted into a proton (composed of 2 up quarks and 1 down quark)
• In other words a down quark is converted into an up quark
• This “weak” interaction is mediated by a W-
intermediate vector boson• Recall the 4 fundamental forces:
– Gravity– Electromagnetism– Strong nuclear– Weak nuclear
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Positron emission
• A way to deal with excess protons• Competes with electron capture• Proton converted into a neutron plus a positron plus an
(electron) neutrino• Recall that neutrinos exist in 3 flavors:
– ne, nm, nt
• Same conservation rules:– Mass (baryons)– Charge– Matter/anti-matter– Lepton number– Energy– Momentum
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Welsh JS. Am J Clin Oncol 2007;30: 437–439)
-
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At a more fundamental level
• A proton (uud) is converted into a neutron (ddu)
• An up is converted into a down
• Recall that up quarks carry +2/3 charge while down quarks carry -1/3
• Thus a +1 baryon (proton or uud) is converted into a charge zero baryon (neutron or udd)
• Mediated by W+ boson
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Electron capture
• AXz + 0e-1 AyZ-1 + ne
• 1p1 + 0e-1 1n0 + ne
• EC can only happen if:
• MA - MB > W/c2
– (MA - MB)c2 > W
• For 2 neighboring isobars on the periodic table, EC can occur only when atomic mass difference between parent and daughter exceeds mass-energy equivalence of lowest electron binding energy of parent
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Electron capture
• Electron capture and positron emission both solve the problem of excess protons
• Competing nuclear mechanisms
• Positron emission wins out in low-Z elements– e.g. 11C, 15O, 18F
• EC wins out in high-Z elements– e.g. 131Cs, 125I, 103Pd
– Due to Coulombic attraction pulling electron cloud closer to nucleus
– Some can do both
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A variety of radiation can follow electron capture
• Loss of an electron leaves a vacancy that is filled by cascading electrons from higher energy shells leading to characteristic x-rays
• Instead of characteristic x-rays, Auger and Coster-Kronig electrons can be emitted
• Capture of an electron leaves the nucleus in an excited state
• (Prompt) gamma photons or conversion electrons (via internal conversion) can be emitted from the nucleus
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Coster-Kronig and Auger electrons follow electron capture
(compete with characteristic photons)
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Conversion electrons can also follow electron capture
(compete with gamma photons)
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Conversion electrons can also follow electron capture
(compete with gamma photons)
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Isomeric transition
• Excited nuclear state decays to ground level• No change in Z, N or A• Typically refers to metastable states transitioning
to lower energy (as opposed to “prompt” gammas)
• Results in emission of a gamma ray• Example: 99mTc 99Tc + g (~140keV)
– Note: reason for metastability is difference in parent (+1/2) and daughter (+9/2) spin states
• Competing with gamma photon production is internal conversion
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Internal conversion: “Conversion electrons”
• Instead of a g emanating from the metastableisomer nucleus, an e- is ejected from the electron cloud
• Can be thought of as an “internal photoelectric effect” – a virtual g interacts with and ejects an electron
– More precisely, a 1s (or 2s or 3s) orbital e-wavefunction interacts with the nucleus and the excitation energy is directly transferred to the electron
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Internal conversion: “Conversion electrons”
• Explains how half-life of Tc-99m can differ based on chemical environment…
• If electrons are less available (because of chemical bonds pulling them away) conversion is less likely and the branching ratio and half life are affected!
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Internal conversion: “Conversion electrons”
• More likely with high Z:
– internal conversion ~Z3
– Conversion coefficient: (# of de-excitations via e) / (# of de-excitations via g)
• Technically NOT beta decay since the electron originates from the orbital cloud rather than the nucleus
– Also, conversion electrons are monoenergetic!
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Internal pair production
• Also competes with gamma emission
• An electron/positron pair emitted instead of a gamma photon or a conversion electron
• Can happen if energy of the decay >2x the rest mass of the electron:
• Eg > 2mec2 (i.e. 0.511MeV x 2 = 1.02 MeV)
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Internal pair production
• “…although 90Y has been traditionally considered as a pure β– emitter, the decay of this radionuclide has a minor branch to the 0+ first excited state of stable 90Zr at 1.76 MeV, which is followed by a β+/β– emission...”
• …it was proposed to use this pair production in radiation therapy in order to assess 90Y biodistribution by (PET)…
• D'Arienzo M. Emission of β+ Particles Via Internal Pair Production in the 0+ – 0+
Transition of 90Zr: Historical Background and Current Applications in Nuclear Medicine Imaging. Atoms. 2013; 1(1):2-12.
• Selwyn, R.G.; Nickles, R.J.; Thomadsen, B.R.; DeWerd, L.A.; Micka, J.A. A new internal pair production branching ratio of 90Y: the development of a non-destructive assay for 90Y and 90Sr. Appl. Radiat. Isot. 2006, 65, 318–327.
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Double beta decay
• 35 naturally occurring isotopes are capable of double beta decay
– 2 neutrons in the nucleus are converted into 2 protons
– 2 electrons (and two electron antineutrinos) are emitted
• For double (or single) beta decay to occur, the final nucleus must have a larger binding energy than the original nucleus
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• For some nuclei, (e.g. Ge-76), the nucleus one atomic number higher (As-76) has a smaller binding energy, preventing single beta decay
• However, the nucleus with two greater protons (Se-76) does have a higher binding energy
• so double beta decay of Ge-76 is allowed
Double beta decay
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Double electron capture
• 35 naturally occurring isotopes are theoretically capable of double electron capture– 2 protons in the nucleus are converted into 2
neutrons by capturing two orbital electrons (and forming two electron neutrinos)
• Z drops by 2 but A remains the same
• Only experimentally confirmed for Ba-130– (by detection of predicted daughter product Xe-130
in geological samples)
– T1/2…. 1021 years!
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Postassium-40 decay• A primordial radionuclide• T1/2 ~1.248 × 109 years• Major endogenous radionuclide• 0.012% (120 ppm) of all potassium• 70 kg body contains ~160 total grams K and ~19mg 40K
– 0.00012 x 160g = 0.0192 g of 40K
• Decay continuously produces about 4,900 Bq• Quite unusual• THREE modes of decay
– 88.8% beta minus– 12.2% electron capture– Tiny fraction (~0.001%) via positron emission
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Beta emitters for bone metastases
• Strontium-89
• Phosphorus-32
• Samarium-153
• Holmium-166
• Rhenium-188
• Tin-117m
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Phosphorus-32
• Historical use dates back to 1940’s
• Half-life = 14.3 days
• Max beta energy = 1.71 MeV
• Avg beta particle energy = 0.693 MeV
• Significant marrow toxicity
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• Half-life = 50.5 days
• Ebmax= 1.463 MeV (100%)
• Max range in tissue: 8 mm
• Average soft-tissue range 2.4 mm
• Decays to 89Y (a stable isotope) with emission of a
negative beta and an electron antineutrino
Strontium-89
Silberstein, et al. Society of Nuclear Medicine Procedure Guideline for Palliative Treatment of Painful Bone Metastasesversion 3.0 Jan, 2003
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Strontium-89
• Biochemically acts as a calcium analogue
• Used as a chloride salt (89SrCl2)
• Can be produced via:
– 88Sr (n,g) 89Sr
–89Y (n,p) 89Sr
Possibly via nuclear transformations of the fission products in the decay chain 89Se→89Br→89Kr→89Rb→89Sr
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Samarium-153
• Beta and Gamma emitter
• Beta: 640 keV (30%)
710 keV (50%)
810 keV (20%)
• Gamma: 103 keV (29%)
70 keV (5.2%)
97 keV (1.3%)
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Samarium-153
• Produced in high yield and purity by neutron irradiation of isotopically enriched samarium oxide (152Sm2O3)• 152Sm (n, g) 153Sm• 152Sm2O3 + 1n ------> 153Sm + g
• (specific activity might be hindered by this approach?)
• Physical half-life = 46.3 hours (1.93 days)• Complexed with ethylenediamine
tetramethylene phosphonate (EDTMP or lexidronam)
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Holmium-166
165Ho (n,g) 166Ho
Chelated to a phosphonate with skeletal uptake similar to Tc-99m-MDP
Primarily a beta emitter with a relatively high energy (Emax = 1.85 MeV) Eβavg = 0.67 MeV
May be useful for larger tumorsHalf-life of 26.8 hours
Relatively high dose-rate
Minor gamma component (81 keV) suitable for imaging
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Re-188
Physical half life 17.00 h
Maximum beta energy (abundance)
2120.4 keV (71.1%)
1965.4 keV (25.6%)
Gamma energy (abundance) 155.0 keV (15%)
Maximum penetration in tissue
10 mm (average 3.1 mm)
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Penetration of g-rays, β Particles and α Particles into Bone and Marrow
Figure from Brady D, Parker C, O’Sullivan J. Bone-Targeting Radiopharmaceuticals Including Radium-223. The Cancer Journal. 2013;19:71-78. Copyright © 2013 The Cancer Journal. Reprinted with permission from Lippincott Williams and Wilkins/Wolters Kluwer Health.
93
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Comments• Some gamma photons are low energy
• EC agents like Pd-103– Therapeutic radiation = gamma photons and
characteristic x-rays
– Dose distributions not much different from hi-energy betas
• Some electrons are VERY short range – shorter than alpha particles
• Auger and Coster-Kronig electrons
• High-LET/RBE!
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Beta-Emitting Radionuclides Used in Brachytherapy
• Strontium-90
• Phosphorus-32
• Ytrium-90
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Strontium-90
• T1/2 = 29 years (28.78y)
• Beta decay to Y-90 and Y-90m with a maximum energy of about 0.5 MeV
• Classic fission byproduct
• Therapeutic radiation is primarily from 2.27 MeV betas from Y-90
• Pterygium eye applicators and coronary brachytherapy
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Ytrium-90
• T1/2 = 64.1 hours
• Beta decay into Zr-90 with a maximum energy of 2.28 MeV
• Range: 1.1 cm
• Used in microspheres (resin and glass) for liver microsphere brachytherapy (“radioembolization”)
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Phosphorus-32
• T1/2 = 14 days (14.262d)
• Beta decay to Sulfur-32 with a maximum energy of 1.71 MeV
• 32P15 ----> 0e-1 + 32S16
• Average beta particle energy = 0.693 MeV
• Intracavitary applications (colloidal)
• Slightly more limited penetration than Sr-90
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Electron Capture Radionuclides Used in Brachytherapy
• Palladium-103
• Iodine-125
• Cesium-131
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Pd-103
• Half life = 17.0 days
• Avg 0.021 MeV (21 keV) x-rays
• 103Pd + e-
103Rh* + ne
• Excited 103Rh emits characteristic X-rays, gamma photons, conversion electrons and Auger electrons
• In the encapsulated “seed” form only the photons are of clinical relevance
• Photon energy range: 20-23 keV
• Average ~21 keV
• HVL 0.004 Pb
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I-125
• T1/2 = 60.1 days
• 125I + e-
125Te* + ne
• Internal conversion 93% of time (yielding 27.0 keV and 31.0 keV x-rays; avg 28.5 keV) and produces a prompt gamma ray (35.5 keV) 7% of time
• Average 0.028 MeV
• HVL 0.025 mm Pb
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Cs-131
• T1/2 = 9.689 days
• 131Cs + e-
131Xe* + ne
• Excited Xe-131 emits characteristic x-rays
• 4-34 keV photons
• Most prominent peaks in 29-34keV range
• Average 30.4 keV
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Photon-Emitting Radionuclides Used in Brachytherapy and Teletherapy
• Cesium-137
• Iridium-192
• Gold-198
• Radium-226
• Cobalt-60
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Co-60
• Half life 5.263 yrs
• Beta decay 60Co 60Ni + b- + g
• Principal gamma rays produced: 1.17 MeV, 1.33 MeV
• Average gamma energy = 1.25 MeV
• Beta: 0.32 MeV (99%) and 1.48 MeV (1%) Emax
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Co-60 decay scheme
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Radium-226
• T1/2 = 1600 years
• Alpha decay to Radon-222 and down to Pb-206 but the photons are what are used clinically
• 78 g rays from Ra-226 and decay products
• Energy ranging from 0.184 MeV - 2.45 MeV
– Average 0.83 MeV
• HVL 14 mm Pb
• 0.5 mm Pt encapsulation for beta particle filtering
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Gold-198
• T1/2 = 2.7 days
• Beta decays to Hg-198
• 0.412 MeV photons
• Nearly monoenergetic
• Also emits beta particles (maximum energy 0.96 MeV)
• These electrons are absorbed by the 0.1mm thick platinum wall of the seed)
• HVL = 2.5 mm Pb
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Cesium-137
• T1/2 = 30.07 years
• Beta decay to Ba-137m
• 662 keV photons
• Another classic fission product
• HVL 5.5 mm Pb
• Stainless steel encapsulation
• Less shielding than Ra-226
• Typically needs replacement after 7 years
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Iridium-192
• T1/2 = 74 days (73.831d)
• Beta decay to excited states of Pt-192
– AND
• Electron capture to Os-192
• Complex energy spectrum
• Average photon energy ~0.38 MeV
• HVL 2.5 mm Pb
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Conclusions
• Isotopes are fun