Cobalt

In 1938, 60Co was discovered by John Livingood and Glenn Seaborg at the University of California-Berkley; Livingood activated samples in a cyclotron and Seaborg separated the elements/isotopes of interest.

After World War II and the atomic bombs at Hiroshima and Nagasaki, scientists at the Canadian National Research Council identified 60Co as a possible useful radioactive source for radiation therapy. This was due to the fact that in 1947 a heavy water reactor facility was available at Chalk River that made possible the production of large quantities of 60Co.

5.3.1 Teletherapy

60Co as a teletherapy source was considered as an alternative to both deep x-ray therapy tubes and sources of some 5 to 10 gm ^6Ra for treating deep-seated tumors. The economics for the new 60Co was a major consideration. A 60Co teletherapy machine could be constructed with costs for the required activity of the 60Co source of 50,000 U.S.$. For an equivalent 226Ra source the costs were a factor of 1000 higher.

In 1949, the Canadians started two independent 60Co teletherapy projects: in Ottawa and in Saskatoon. The first patient was treated on October 27, 1951 by the Eldorado machine in Ottawa and on November 8 of the same year, a patient was treated at the University of Saskatchewan in Saskatoon.

5.3.2 Brachytherapy

In a letter to the journal Science in June 1948 on "Radioactive Needles Containing Cobalt 60", William Myers from the Department of Medicine, Ohio State University,2 described studies with 60Co started in October 1947.

For the production of 60Co radioactive needles, an alloy wire composed of 45% cobalt and 55% nickel, the so-called cobanic, was used in order to overcome the machining difficulties related to pure cobalt. In this work, needles of a very small diameter, 1.0 mm, are described. The half-value thickness of the gamma irradiation of this type of source was 0.41" (10.41 mm) Pb.

Myers also stated that the expected benefit when using 60Co sources when compared to 226Ra should be that the 60Co radioactive wires can be bent to conform to the shape of tumors which are in a difficult anatomical environment (e.g., bones) and that there is no danger of loss by leaks or breakages. This was the worldwide first use of 60Co radioactive needles for cancer therapy.

The initial design of the 60Co sources was in the form of needles and were essentially a copy of existing 226Ra needles. Due to its high specific activity, 60Co (see Table 5.2) is appropriate for fabrication of small high-activity sources and has been mainly used to replace 226Ra for gynecological brachytherapy. It is more expensive to produce than 137Cs but is still in use in some centers in the developing world. However, because of the 60Co half-life of 5.27 years it has important cost advantages over, for example, 192Ir sources, which because of their much shorter halflife of 73.8 days have to be replaced far more often. In addition, all the associated bureaucratic problems of crossing customs boundaries and ensuring a rapid delivery of 192Ir sources is not relevant for the longer lived 60Co.

60Co undergoes |3" decay to excited states of 60Ni (see Figure 5.3). The de-excitation to the ground sate of 60Ni occurs mainly via emission of 7-rays where there are two dominant energy lines of 1.1732 and 1.332 MeV,

60Co Qft-= 2.89239 MeV

P3, Eft

P3, Eft

0Ni*

| y =0.347 MeV (8x10 3%)

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