Metallic Cobalt 60 (Co-60)
Cobalt 60


Cobalt-60 (60Co) is a synthetic radioisotope that has been used in both external beam radiation therapy and brachytherapy. 60Co decays via \(\beta\) decay to an excited state of Nickel-60 which then emits two high energy (1.17MeV and 1.33MeV) gamma-rays. It is these energetic gamma rays which are primarily important for use in radiation therapy. The \(\beta\) particle emitted is absorbed by the cobalt metal and the sources encapsulation such that it doesn’t contribute meaningfully to the dose distribution. Cobalt 60 has been largely replaced by other Iridium-192 in high dose rate brachytherapy and by linear accelerators in external beam radiation therapy. Because of it’s high activity and simple gamma spectrum, it is used in the calibration of radiation detectors. Today it is also used in some MRI guided external beam therapy systems Cobalt-60 is able to generate an MV beam without strongly influencing a magnetic field.

Cobalt-60 (Co-60) decay scheme
Cobalt-60 decay scheme

Quick Facts

  • Half life: 5.2714 years
  • Decay Mode: \(\beta ^-\)
    • \(^{60}_{27}Co \rightarrow ^{60}_{28}Ni + e^- + \bar{v}_e + \textrm{gamma rays}\)
  • Mean gamma energy: 1.25MeV (1.17MeV and 1.33MeV)
    • ~10% of the beam is resultant of scattered photons.
  • Lead HVL: 11.0mm
  • Exposure rate constant: \(\Gamma_{\delta} = 13.07 \frac{R \ cm^2}{mCi \ hr}\)
  • External beam source size: 1-2cm2
    • This increases beam penumbra
Comparison of the dose distributions from Co-60 and Ir-192 brachytherapy sources.
Co-60 vs Ir-192 radial dose distribution.

Use in Brachytherapy

Cobalt-60 is was used in brachytherapy in the past but is rarely used today. Compared with Ir-192, Cobalt-60 is more expensive and requires a larger source size for the same dose rate but has the advantage of a longer half life. The dose distribution is more dominated by inverse square law than Ir-192 leading to a slightly shallower dose fall off.

Use in External Beam Therapy

Cobalt-60 is the radionuclide of choice for nuclide based external beam radiation therapy. Compared to other radionuclides used for this purpose (Radium-226 and Cesium-137), 60Co has a greater specific activity and a higher energy. Greater specific activity allows for smaller source size and higher energy allows for a more penetrating and more skin sparing field. Cobalt-60 external beam therapy has recently been incorporated in on-line MR guided radiation therapy programs where it’s ability to generate an MV beam without significantly perturbing the MRI’s magnetic field is a great advantage.