The physical aspects of a cesium teletherapy source.

A 2,000 curies cesium 137 teletherapy source has been designed for use in a theratron. The large source diameter, 1 1/4 inches, required by the low specific activity of the cesium chloride, 22.4 c/g, made additional collimation a necessity. The low output of the source, 8 trmin. at one meter, forced a change of r/eatment distance from 75 cm. to 50 cm. source skin distance. The electronic equilibrium thickness of tissue equivalent material was found to be 1.4 mm. Percentage depth dose, exposure dose, and absorbed dose rates are given for 22 treatment fields. Comparison of cesium 137 and cobalt 60 isodose curves is made for 3 typical field sizes, with the finding that in general the shapes of the curves are quite similar, but that the cobalt holds a definite advantage over the cesium from the standpoint of its greater depth dose. (auth)