The cobalt bomb that treated cancer
Before 1951, deep radiotherapy meant radium — scarce, expensive, and weak enough that treating a tumour more than a few centimetres down was barely possible.
Harold Johns at the University of Saskatchewan built the first cobalt 60 teletherapy unit that year, and a second machine treated a patient in London, Ontario, within weeks. The two are still argued over for priority.
What mattered was the economics. A cobalt source could be made in a reactor for a fraction of the cost of radium, and its gamma rays at 1.17 and 1.33 megaelectronvolts reached tumours that radium could not.
⚠️ The machines were called cobalt bombs, a name that has aged badly. It referred to the shielded head, not to any weapon.
Within a decade cobalt teletherapy was standard in hospitals across the world, and cancers of the cervix, head and neck became treatable in places that could never have afforded radium.
What replaced it, and where it did not
Linear accelerators have displaced cobalt in wealthy countries. They produce X-rays on demand, switch off completely, and shape the beam more precisely.
Cobalt kept two advantages. It needs no power supply beyond a motor to move the shield, and it needs no team of physicists to keep it calibrated. In hospitals with unreliable electricity or thin technical staff, a cobalt unit works when an accelerator does not.
The World Health Organization has argued both sides of this. Cobalt is more robust and cheaper to run; it is also a source that cannot be turned off and must be secured for its whole life and disposed of afterwards.
⚠️ The Gamma Knife, used for brain tumours, is still cobalt — around 200 small sources arranged so their beams converge on a single point. Nothing else delivers that geometry as simply.
Sterilising the disposable world
The larger use by volume is not medical treatment but sterilisation.
Syringes, surgical gloves, sutures, catheters, implants — most single-use medical equipment is sterilised by passing it, already sealed in its packaging, through a gamma field from a cobalt 60 source.
Heat would melt the plastic. Chemicals would leave residue and require the package to be opened. Gamma rays pass straight through the box and break the DNA of anything alive inside.
The same facilities irradiate food: spices, which otherwise carry a heavy bacterial load, and some meat and produce. The process is endorsed by the WHO and mandatory for some imports.
⚠️ It does not make the food radioactive. The food never contacts the source, and gamma rays at these energies cannot alter a nucleus. The objection that persists in public debate is not supported by the physics.
Five years, and why that suits
Cobalt 60 has a half-life of 5.27 years, which is short enough to be inconvenient and long enough to be practical — and the combination is what makes it useful.
A source loses about 1 % of its strength a month. A radiotherapy unit therefore needs its treatment times recalculated regularly and its source replaced roughly every five to seven years, which is a running cost.
Against that, a spent source becomes harmless on a human timescale rather than a geological one. After fifty years it retains about one part in a thousand of its original activity. Caesium 137, at 30.1 years, takes three centuries to reach the same point.
⚠️ This is the trade behind the choice between them. Cobalt is more energetic, more penetrating, and decays away; caesium is cheaper, weaker, and lasts. For a source that will one day be abandoned — and some always are — the one that decays is the safer failure.
The sources that get lost
A spent teletherapy source is a metal cylinder a few centimetres long. It looks like scrap. It emits continuously, silently, and cannot be switched off.
In January 2000, a disused cobalt 60 source from a hospital in Samut Prakarn, Thailand, was sold as scrap. Workers at a scrapyard cut into the housing. Ten people were seriously irradiated and three died.
The pattern repeats: Ciudad Juárez in 1983, where a source ended up melted into reinforcing steel that was built into houses; Mayapuri in Delhi in 2010, where a university sold an irradiator to a scrap market.
⚠️ The IAEA calls these orphan sources and maintains a register of them. The danger is not that cobalt 60 is unusually hazardous — it is that a source outlives the institution that bought it, and institutions close.