The basic physical properties of the two gamma-emitting radionuclides (cobalt-60 and cesium-137) that have proven useful in external beam teletherapy and a potential source for teletherapy (europium-152) are listed in Table 7-1. Of the three radionuclides, cobalt-60 is the most widely used, because it offers the most practical approach to external beam radiotherapy, considering the energy of emitted photons, half-life, specific activity, means of production, and safety. Cobalt-60 and europium-152 come in metallic form encapsulated in a special source container, while cesium-137 is used in the form of cesium-137 chloride encapsulated in a special source container.
The use of cesium-137 for external beam radiotherapy was discontinued during the 1980s despite its attractive half-life. There are two reasons for this: (1) security of cesium sources is a major concern and, in addition, (2) cesium-137 has a relatively low specific activity. The low specific activity implies a relatively low source output and large source diameter; this effectively precludes a teletherapy machine SAD larger than 50 cm, yet in modern radiotherapy a minimum SAD of at least 80 cm is the accepted norm. Note that cesium-137 is still used in blood irradiators and research irradiators because in these machines the distance between the source and the irradiated object is relatively short, of the order of only 10 to 25 cm.
A cobalt-60 teletherapy source is typically a cylindrical stainless-steel capsule containing many hundreds of tiny, high-activity cobalt metal pellets and sealed by welding. A double-welded seal is used to prevent any leakage of the radioactive material from the source container. The typical diameter of the cylindrical teletherapy source is between 1 and 2 cm, the height of the cylinder is about 2.5 cm. For a given activity, a smaller source diameter yields a smaller physical penumbra,1 making for a sharper beam edge; but higher specific activity sources are more expensive. Often a diameter of 1.5 cm is chosen as a compromise between the cost and penumbra size. The cost of a cobalt-60 teletherapy source is on the order of $200/TBq ($7.5/Ci).