Radiation Oncology/Physics/Isotopes

A listing of useful isotopes in Radiation Oncology

See also: Unsealed sources

Periodic table

Overview

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

Phosphorus

P, Z = 15

• P-32

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

Cobalt

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)

Strontium

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

Yttrium

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

Palladium

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.

Iodine

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

Cesium

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.

Iridium

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)

Gold

Au, Z=79

• Au-198

Radium

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