Tissue isodose curves for beta surface applicators.

Abstract

Beta-emitting surface applicators have been in clinical use for many years. The inconsistencies between physical dosage and clinical results have been such, however, that it has, for practical purposes, fallen upon each clinician to evaluate his own applicator on the basis of his personal experience. Poor correlation has resulted when attempts were made to compare results from one applicator with those from another, or from one clinic with those from another. Hughes (1), impressed by these difficulties, asked the question, “How can an ophthalmologist or radiologist be certain of the clinical effect to be expected from a specific applicator?” The strontium-yttrium-90 applicators are in common usage today in many clinics and offices. The half-life of strontium 90 is given as twenty years (2). In some cases of recalibration of these applicators, the dose rate was found to have decreased by as much as 70 per cent of one half-life in one year. This rapid change in the strength of an applicator with a long half-life can be explained only by inconsistencies in the methods of calibration. Yet the same methods of calibration are in use today. In clinical medicine we must accept occasional inconsistencies in therapeutic results because of normal biological variation. However, we have felt, as have others, that the lack of consistency of results in the case of topical beta therapy has been significantly beyond the limits of normal variation. It is evident that a fundamental error exists, either in the hypothesis, in the technic, or in the interpretation of the experimental data used in support of the hypothesis. In an effort to clarify the basic concepts of the dose delivered to tissue by a surface beta applicator, we must first understand the steep gradient of absorption of particulate radiation. Any surface beta applicator has a finite area, usually 1 sq. cm. or less, with a uniform distribution of radioactive material over its surface. Assume that such a beta applicator, emitting 100,-000 beta particles each second, is placed in contact with an area of tissue. Since these particles may possess energies ranging from a few hundred electron volts up to a maximum value of several million electron volts, it is evident that they will penetrate to significantly different depths in the tissue. If we assume that 100 per cent strike the surface, 40 per cent penetrate to a depth of 1 mm., 16 per cent to a depth of 2 mm., etc., the resulting density of the radiation within the tissue would be as shown in Table 1. A practical example is shown in Figure 1. This figure demonstrates a hypothetical distribution of beta radiation density within a 1.0 c.c. block of tissue under the applicator. It is apparent that 100 per cent of the radiation is absorbed within the 1.0 c.c. of tissue.