LII:Radiation Oncology/Physics/Isotopes

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A listing of useful isotopes in Radiation Oncology

See also: Unsealed sources

Periodic table


Listed in order of increasing atomic number (Z).

Sealed Sources
Radionuclide Source Half-life Gamma Energy HVL (mm Pb)
Co-60 Neutron absorption 5.3 years 13.1 Gamma: 1.25 MeV 11
Sr-90 Fission product 28.8 years   Beta (Sr-90): 546 keV

Beta (Y-90): 940 keV

Pd-103 Neutron absorption

Proton absorption

17 days 1.5 X-ray: 21 keV 0.008
I-125 Neutron absorption 60 days 1.4 X-ray: 28 keV 0.028
Cs-131   9.7 days   X-ray: ~30 keV  
Cs-137 Fission product 30 years 3.3 Gamma: 662 keV 6.5
Ir-192 Neutron absorption 74 days 4.7 Gamma: 380 keV 2.5
Au-198 Neutron absorption 2.7 days 2.4 Gamma: 412 keV 2.5
Ra-226 Uranium ore 1622 years 8.2 Gamma: 830 keV 8

Unsealed Sources
Radionuclide Source Half-life Energy
P-32 Neutron absorption 14.3 days Beta: 695 keV
Sr-89 Fission product 50 days Beta: 1.46 MeV
Y-90 Sr-90 daughter 64 hours Beta: 940 keV
I-131 Fission product 8 days Beta: 806 keV
Sm-153 Neutron absorption 47 hours Beta: 810 keV

  • Preparation overview
    • Spent nuclear reactor: Cs-137, Sr-90
    • Neutron bombardment in nuclear reactor: Co-60, Ir-192
    • Proton bombardment in cyclotron: C-11, N-13, O-14 (short half-life positron emitters)
    • Elution of metastable daughter: Tc-99m


P, Z = 15

  • P-32

Half life: 14.29 days Decay: B minus to S-32 B energy: 0.5MeV


Co, Z = 27

  • Co-60: Half life 5.263 yrs. Gamma energy 1.25 MeV (avg)
Beta decay, 60Co->60Ni (stable). Gamma rays produced: 1.17 MeV, 1.33 MeV. Beta: 0.32 MeV (99%) and 1.48 MeV (1%) Emax.
Clinical use: cobalt teletherapy, brachytherapy (remote afterloading), Gamma Knife radiosurgery
Clinical form: teletherapy source (1-2 cm diameter, Co-60 in stainless steel capsule), brachytherapy (in stainless steel capsule)


Sr, Z = 38

  • Sr-89 Half-life 50.6 days
Source: byproduct of nuclear fission
Energy beta 1.46 MeV
  • Sr-90 Half-life 28.5 years; in secular equilibrium with daughter Y-90 (half-life 64 hours)
Source: byproduct of nuclear fission. Present in significant amounts in spent nuclear waste
Energy: beta (Sr-90) 546 keV, beta (Y-90) 940 keV
Dose rate: ~1 Gy/sec at surface, ~5 cGy/sec at 4mm (lens)
Clinical use: As applicator for superficial skin and ophthalological cancers
Toxicity: Seen by body as calcium, and deposited into bone. Bone marrow toxicity


Y, Z = 39

  • Y-90: pure beta emitter, average energy 0.94 MeV, tissue penetration 2.5 mm, maximum range 1.1 cm. Half life 64.2 hrs.
Clinical use: hepatic microsphere therapy; Zevalin in NHL


Pd, Z = 46

  • Pd-103: Avg 0.021 MeV (21 keV) x-rays. Half life 17.0 days.
Produced from stable Pd-102 by absorption of a neutron
Decays via electron capture. 103Pd -> 103Rh, which undergoes internal conversion and emits characteristic X-rays (range: 20-23 keV). Auger electrons also emitted.
Clinical use: interstitial brachytherapy (prostate)
Clinical form: seed model 200 (Theragenics Corp) - 4.5 mm x 0.8 mm; two graphite pellets plated with Pd-103 in titanium tube. Lead marker between pellets allows for identification on X-rays.


I, Z = 53

  • I-125: Avg 0.028 MeV (28.5 keV) x-rays. Half life 59.4 days.
Produced from Xe-124 by absorption of a neutron
Decays by electron capture. 125I -> 125Te, which undergoes internal conversion 93% of time (to 27.3 keV, 31.4 keV x-rays; avg 28.5 keV) and produces gamma ray (35 keV) 7% of time
Commercially available as small "seed" sources
  • I-131
Clinical use: thyroid cancer, Bexxar for NHL


Cs, Z = 55

  • Cs-137: gamma 0.662 MeV. Half life 30.07 yrs.
Produced from fission of uranium U-235 in nuclear reactors. It is one of the major fission byproducts, and accounts for most of the radioactivity in spent radioactive fuel (and from nuclear contamination after accidents such as Chernobyl). Some is extracted for commercial uses
Decays by beta decay 137Cs -> 137Ba. Gamma rays from metastable Ba-137 produced (93.5%) - 0.662 MeV
Decay correction ~2%/year
Clinical use: interstitial and intracavitary brachytherapy
Clinical form: needles or tubes; ceramic microspheres or powders encapsulated in stainless-steel. Beta and low energy x-rays absorbed by tube, so source acts as a pure gamma emitter.
Lifecycle of Cs-137 in the U.S.


Ir (Z=77)

  • Ir-192: gamma, avg E 0.37 MeV. Half life 74.2 days
Produced from stable Ir-191 by absorption of a neutron in a nuclear reactor
Decays by beta decay (95%) 192Ir->192Pt and electron capture (5%) 192Ir->192Os. Complex energy spectrum (0.1 - 1.1 MeV, average 0.37 MeV)
Decay correction ~1%/day
Clinical use: interstitial brachytherapy (HDR)
Clinical form: alloy of 30% Ir and 70% Pt. Thin flexible wires. Nylon ribbons containing Ir-192 seeds
Available in variety of air kerma strengths (1.4-7.2 U commonly, up to 72 U)


Au, Z=79

  • Au-198


Ra, Z = 88

  • Ra-226: gamma, 1.2 MeV avg. Half life 1600 yrs
No longer as prevalent for medical use due to decay into radon gas


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