Cobalt: From Goblins Ore to Cancer Cure

Cobalt is fairly uncommon metal in nature, and even more uncommon in nature is the color blue. Cobalt in its pure form is a beautiful blue metal. This lead cobalt to be a very useful blue pigment, right back to the Egyptian and Persian ages in jewelry and paintings. Cobalt has also been found in the ruins of the ancient city of Pompeii. A transition metal with modern uses in magnets, high tech turbines, and other industry, and still used as a pigment in ceramics and paints.

The name Cobalt comes from “kobold”, German for goblin. Medieval miners considered it “goblins ore” and was troublesome to deal with. It released toxic vapors when smelted. and wasn’t of much used due to its brittle nature.

Spent radioactive material from a reactor showing the blue glow of Cerenkov radiation

But I am not here to talk about the naturally occurring Cobalt. I want to talk about the man made radioactive cobalt forged in nuclear reactors. This energetic radioactive cobalt 60 was to change the field of radiotherapy. Early radiotherapy had to do with low power x-rays. Then, with increasing technology they were able to increase the power into the 100’s of kilovolts, but still there was a desire to use higher energy x-rays. These higher energy x-rays increasing the skin sparing effect. Lowering the skin dose was a major goal of early radiotherapy, since the skin dose limited the dose that could be delivered to the tumor. Cobalt 60 was a material that would deliver the higher energy photons everyone was after in the form of Gamma rays.

The useful Cobalt for radiotherapy, Cobalt-60, is created using a technique called neutron bombardment. The naturally occurring Cobalt 59 is bombarded with neutrons, some of these neutrons are then absorbed by the nucleus of the atom. This transforms it to Cobalt 60. This process can take 5 to 10 years to make the sample of cobalt radioactive enough to make it useful in radiotherapy. That’s a long time to leave some metal inside a reactor.

The number 59, and 60, refer to the number of neutrons in the nucleus of the cobalt atom. To make this cobalt you need a large source of neutron. The only place on earth where this is available is inside a nuclear reactor. Neutrons are continuously whizzing around from the fission of nuclear material. These reactors are special reactors called Neutron Flux reactor and are generally research based reactors, not power reactors.

Now, this extra neutron in Cobalt 60 makes it unstable- it wants to release energy to become more stable again. This is done through the production of gamma rays. The gamma rays that are given off have energies of 1.17 and 1.33 MV. This lead the radiotherapy world out of the kilovoltage error and into the megavoltage error.

Cobalt 60 machines were first used for patient treatments in 1951 in Canada, and although for some years earlier there had been linear accelerators (Linac) but their uptake was slow. Cobalt 60 machines became mainstream quickly due to their simplicity and affordability. They dominated radiotherapy, or teletherapy as it was more commonly know then. The machine itself consisted of only three main components, the head, the gantry, and the treatment table.

The treatment head had three main functions, to shield the source when not in used, to allow the source to be moved into position to begin the exposure, and to collimate the beam into rectangular shaped fields. To shield the source when not in use the head had to be made from a lot of steel and lead. I have been told that as all the radiation is being absorbed in the head it was usually warm to the touch, especially after a new source had been installed.

The gantry allowed the head to be rotated around the patient. This allows for the optimization of individual patient treatments by targeting the tumors from different directions. The gantry had to be sturdy as the source had to be pointed at the same location from each angle.This point is called the isocenter and it isn’t as easy you think. Insuring that the machine points to the same point from every angle is difficult when you consider the weight of the steel and lead.

The treatment table serves one function, to support the patient. The treatment table is sometimes referred to as a treatment couch, however it is not very couch like. It is a flat hard service. You may think this is cruel to patients making them lie on a hard flat service for their treatment. There is a reason for this though. The patient has to be aligned exactly as they were when simulated. The easiest way to do this is to have them lye on a hard surface than a soft squishy one.

Once the technology of linear accelerators, of Linac as they are known, has developed to produce affordable and reliable machines the number of cobalt machine started to decline. It was known that the newer Linacs had better geometric accuracy, and produce higher energy x-ray beams. These higher energies are more penetrating and allow for deeper targets to be treated.

An early Cobalt 60 Machine

Asides from these points, Linacs had the advantage of being computer controlled. The rate at which dose was delivered, the energy or x-rays and other things could all be controlled using computed. Unlike with Cobalt, which was always “on” as its radioactive source would always be decaying. The need to replace this source every 5 years was another one of its downfalls. Having to deal with highly radioactive material is not desirable.

The overall simplicity of cobalt machines have now made these machines suitable for the third world and developing countries. They are very reliable, reduced their maintenance requirements and downtime. However even this is being challenged by Linear accelerators as they become ever more affordable. Although Cobalt 60 is still used in one area of radiotherapy, radiosurgery using the Elekta Gamma Knife.

Bonus: While I was researching Cobalt I came across some other facts that I though might be interesting.

• Cobalt is an essential trace nutrient required for our health. It is needed to form vitamin B12, which is used in blood formation.
• The Democratic Republic of Congo currently extracts roughly 40% of the world’s cobalt supply each year.

If you want to read more about how Cobalt 60 changed the radiotherapy world there is a great book called Cobalt Blues: The Story of Leonard Grimmett, the Man Behind the First Cobalt-60 Unit in the United States, which describes the development of the first Cobolt 60 treatment unit in the united states.

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Originally published at www.radicalradiationremedy.com.