Tissue Dosage from Interstitial Radiation

Abstract

THE calculation of tissue dosage from gold radon seeds in terms of a reproducible standard or unit is essential not only that we may know the dose administered to the tumor itself, but also to the surrounding healthy tissue. This is particularly important when external radiation is applied in addition to interstitial implants. Pending the establishment of a standard unit, we have in the past endeavored to relate the intensity from seeds (1), from intracavitary radiation (2), and from surface application of radium tubes, to the degree of reaction of the skin (erythema) produced by applying a 100-milligram tube of radium for 450 milligram-hours at a distance of one centimeter. These conditions are easily reproduced. The specifications of this tube are as follows: Length over all 21.5 millimeter Outside diameter 4 millimeters Wall thickness 1 millimeter of platinum Radiating length 19.5 millimeters When applying this tube to the skin, a spacer of 8 millimeters of wax was used, making the distance from the center of the radium to the skin exactly one centimeter. This has been adopted provisionally as our standard. The present paper is an elaboration of the previous communication on the calculation of tissue dosage from radon seeds, presenting in chart form the relation between the dose expressed in the usual terms of millicurie-hours and the previously described erythema dose, for lesions of various diameters. In addition, this paper concerns itself with the dosage and distribution of radiation from needles of various active lengths as related to the above-mentioned standard. The relation between intensity and radiating length expressed as a system of factors was published in a previous paper. Many additional measurements of radiating lengths, ranging from 10 to 100 millimeters at a distance of 1 centimeter with wax as the spacer, have been made with a different instrument, a condenser type r meter, fitted with two ionization chambers, one having a volume of 2 cubic centimeters and the other a volume of 1/7 cubic centimeter. The values obtained from this series of measurements agree closely with the earlier measurements, and are shown graphically in Figure 1. Radon Seeds.—The radon seeds in use at this Institute are 3 millimeters long, with a wall thickness or filtration of 0.3 millimeter of gold. In order to extend the curve to include this radiating length of 3 millimeters, it would be necessary to make measurements at this point. However, due to the low radon content of seeds, variation of length and possible lack of uniformity in the gold wall, it was felt that accurate measurements might not be obtainable directly. Instead, a large quantity of radium enclosed in a lead box with one side open was measured at a distance sufficiently great that the values obtained could be corrected, by means of the inverse square distance law, to one centimeter.