Cobalt Machine

Megavoltage Therapy – Cobalt-60 Unit

The radionuclide answer to megavoltage treatment is the cobalt machine. The cobalt source is made by putting a 2cm pellet of cobalt-59 into a nuclear reactor and irradiated it with neutrons. Because cobalt reacts with air to produce a fine coating of powder, the Co-60 is double sealed in steel. It produced characteristic radiation, called gamma rays or photons, with 2 photons of 1.17 MeV and 1.33 MeV (produces an average energy of 1.25 MeV - given that for bremsstrahlung beams, $mean MeV = {peak MeV\over 3}$ the mean Mev of a 4MV linear accelerator beam is 1.33!). The cobalt radionuclide also produces $\beta$ particles which are absorbed by the steel capsule, and produce some bremsstrahlung x-ray contaminants.

As you would expect, the primary gamma photons also are scattered and produce secondary e- in the source housing causing some contamination. Cobalt has some advantages as a source because it has a high specific activity (very intensely radioactive), a high average photon energy (only Radium does better at ~4MeV), a high output per Curie and less self-absorption. Furthermore it is a plentiful metal and is cheap, and it has no harmful daughter gaseous products like radium because its daughter by decay is another metal.


However it does have the disadvantage of performing very badly when it come to a penumbra. The penumbra refers to the rapid change in dose rate as a function of lateral distance from the central axis of the beam near the field edge, and has a number of components. Classically the geometric penumbra results from a source with a finite size, i.e., not a point source. The geometric penumbra will get bigger as

  • the source size increases,
  • source to diaphragm distance decreases,
  • source to surface or depth distance increases, and
  • is independent of field size.

The other type of penumbra is physical penumbra, which occurs even in the absence of geometric issues. Why does physical penumbra arise? it has to do with the pattern of dose deposition in tissue.

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