Cancer Therapy

1951 was an important year for both the fight against cancer and the emergence of Canada as a leader in the field of nuclear medicine. Previously, ionizing radiation from radium and x-rays had been used to treat cancers inside the body. Radiation therapy works by damaging the DNA (genetic material) of cells. A radiation beam of photons, electrons, protons, or neutrons can be used to disrupt the growth of tumour cells by directly or indirectly ionizing the DNA (deoxyribonucleic acid). The cell with damaged DNA dies during cell division. It is a useful treatment because some cancer cells are more sensitive to radiation than normal cells.

Scientists tried building sources of ionizing radiation called “radium bombs” for teletherapy, but radium radiation was too weak for the job.  It was called a “bomb” because the large amount of heavy metal shielding reminded people of a bomb. High-voltage radiation equipment like x-ray machines could do the trick, but they were expensive, bulky and limited in use to a few cancer centres that had the technical expertise required to maintain and operate them. While scientists around the world had theorized about the possible use of cobalt-60 as a radiation source for the treatment of cancers inside the body, it was Canadian scientists who made this treatment a reality.

Cobalt-60 is a beta emitting radioactive isotope of cobalt-59 and has a half-life of 5.27 years decaying to Nickel-59, a stable isotope. During the beta decay of cobalt-60, gamma rays are produced.

cancer therapy machine
Cancer therapy machine.

The NRX reactor at Chalk River, Ontario was designed to produce radioactive isotopes. In 1949, Dr. Harold Johns of the University of Saskatchewan wrote Wilfrid Bennett Lewis, the director of the National Research Council of Canada’s Atomic Energy Division (now Canadian Nuclear Laboratories), requesting that cobalt-60 produced by the NRX reactor be used in the construction of a prototype cobalt therapy unit. Soon after, two teams of physicists, physicians and engineers working in Saskatoon, Saskatchewan and London, Ontario, respectively, designed two different cobalt-60 apparatuses. One was built under the direction of Dr. Harold Johns at the University of Saskatchewan‘s newly constructed cancer wing which was adjacent to the medical college. There, Dr. Johns and his team collected the depth-dose data (how much ionizing radiation to apply during treatment) which later became the world standard. The second unit was designed by researchers at the University of Western Ontario and built by Eldorado Mining and Refining Ltd. (later to become part of Atomic Energy of Canada Limited).  The Eldorado unit was installed at Victoria Hospital in London, Ontario on October 23, 1951. On October 27, 1951, just four days after installation, the first highly publicized cobalt-60 treatment in the world occurred there. Twelve days later, on November 8, 1951, the first patient was treated in Saskatoon at the Saskatchewan Cancer Commission. With the development of the cobalt bomb, the cure rate for cervical cancer went from 25% to 75%.

Cobalt-60 therapy units contain a small cylinder of cobalt-60 in the treatment head of the apparatus. As the patient lies on the table, a beam of gamma rays passes through a series of collimators and jaws which shape the beam as it is directed at the patient. Because the beam will destroy healthy cells as well as cancerous cells, placement of the beam and the radiation dose must be precisely calculated. Also the treatment head must be rotated at different angles to attack the cancer tumour from different sides without overexposing healthy tissue.

Because of its effectiveness and simple design, for over 50 years the cobalt-60 therapy unit has been one of many tools doctors use in the treatment of cancer. In North America, many of these units have been replaced with newer technology called Intensity-Modulated Radiation Therapy, or IMRT, which uses high-energy electron accelerators directly to produce x-rays. However, because of its cost effectiveness, the cobalt-60 unit remains a good choice in many countries.


cobalt-60 production
Cobalt-60 production.

Cobalt-60 is a beta emitting radioactive isotope of cobalt-59 with a half-life of 5.27 years decaying to nickel-59, a stable isotope. During the beta decay of cobalt-60, gamma rays are produced.

Cobalt-60 is produced by placing rods of cobalt-59 inside the reactor and simply leaving them there for five to 10 years. The rods are then harvested and processed using special equipment. On the right, a technician can be seen checking cobalt-60 while it sits below water in the reactor cooling pool. The blue glow is known as the Cerenkov Effect and is caused by charged particles moving through the water at relativistic energies.

For many years, Canada was the only country that could produce cobalt-60, and today it continues to supply the majority of the world’s demand. As well, over half the cobalt-60 therapy machines and medical sterilizers in the world were built in Canada, treating over half a million patients yearly.


Sandeep Kaur Dhanesar, Conformal Radiation Therapy with Cobalt-60 Tomotherapy, Queen’s University, Kingston, Ontario, April 2008.
Paul Litt, Isotopes and Innovation, MDS Nordion’s first 50 Years, McGill-Queen’s University Press, 2000.
Lewis, Wilfrid Bennett, The Canadian Encyclopaedia
University of Saskatchewan, The Commemoration of the Development of the Cobalt-60,
Lecture by Dr. Doug Boreham, McMaster University, CNA Annual Conference, 2008.
Dr. Harold Elford Johns, Canada Science and Technology Museum,
Jeremy Whitlock, What is Canada’s role in nuclear medicine and isotope production?,
University of Saskatchewan, “Former U of S ‘Cobalt-60’ physicist inducted into Canadian Science and Engineering Hall of Fame”,