Measurement of beta-ray applicators.

Abstract

The beta-ray applicator is a source of very intense beta radiation giving a high radiation dose rate to a small restricted volume of tissue. The ophthalmic applicator, described in detail by Swanberg (10), may have several millicuries of radium-D + radium-E (Ra-D+E), or other beta-emitting isotope, deposited on an area of about 0.25 square centimeter and protected by a thin metal foil. The high activity, small physical size and relatively short range of the particles, which are all advantages in surface treatment, have made the measurement of the dose rate very difficult. As a result large discrepancies have occurred in measurements of the dose rate from Ra-D+E ophthalmic applicators recently produced. Previous measurements (4, 9) have given the surface dose rate as averaged over a large area of the applicator, and the half-value layer averaged similarly. The surface dose rate of a typical ophthalmic applicator (which appears to have a very uniform distribution of the active material) actually varies rapidly along the small diameter (5.6 mm.), as will be shown (Fig. 1). The methods developed by Neary (9) and Spiers (4) are not readily adapted to such small applicators. The apparatus developed here may be used to measure an applicator of any size and for beta energies in excess of about 0.2 mev. Various quantities have been used in the calibration and determination of the medical value of beta-ray applicators. These measures are the quantity of active material, the contact dose rate or average surface dose rate, and the depth dose rate distribution along the center line. All of these fail, in some respect, to indicate the biological effect on tissue at any particular point, especially in the case of very small ophthalmic applicators. Two applicators may be constructed of identical size, and having the same number of millicuries of the same radioactive material, but they may differ in surface dose rate, depth dose rate distribution, or total energy output, as a result of differences in absorption or back-scattering. Thus the quantity of active material, which may be determined when the applicator is being produced, is not a reliable measure of the biological effect or medical value of an applicator. In the cases where the biological effect of beta-ray applicators has been related to the contact dose rate or average surface dose rate, no account seems to have been taken of the lateral dose rate distribution or the fact that the applicator expends practically all of its available energy within a millimeter or two of the surface. Average depth dose rate curves have also been supplied whose shape depends on the distribution of particle energies, and thus in turn on absorption and back-scatter.