The invention of the nuclear reactor in the Manhattan Project during World War II made possible the creation of artificial radioisotopes for radiotherapy.

In 1949, Dr. Harold E. Johns of the University of Saskatchewan sent a request to the National Research Council (NRC) of Canada asking it to produce cobalt-60 isotopes for use in a cobalt therapy unit prototype.

[7] Dr. Glenn T. Seaborg, chairman of the United States Atomic Energy Commission, Nobel Prize winner and former chancellor of the University of California, dedicated the first cobalt facility of the new Radiation Therapy and Nuclear Medicine Wing of the Cedars of Lebanon Hospital on January 11, 1963 supervised by Dr. Henry L. Jaffe, Director of the new department.

[8] The role of the cobalt unit has partly been replaced by the linear accelerator, which can generate higher-energy radiation, and does not produce the radioactive waste that radioisotopes do with their attendant disposal problems.

Cobalt treatment still has a useful role to play in certain applications and is still in widespread use worldwide, since the machinery is relatively reliable and simple to maintain compared to the modern linear accelerator.