Exit Doses for Lung Fields measured by Lithium Fluoride Thermoluminescence

Abstract

Lithium fluoride thermoluminescence dosimetry powder has been used in gelatine capsules of 10 mm length and 4 mm diameter to measure the exit doses in patients having treatment to the thorax by means of 8 MV X-rays. It was found that exit doses for fields passing through the lungs were up to 60% higher than the doses calculated on the assumption of homogeneous unit density tissue; the dose received at the tumor would thus be increased by about half this value. Such increases can be satisfactorily explained because the beam has passed through lung tissue containing air instead of unit density tissue. Where growth or fluid or fibrosis was present in the lung, the excess dose was less marked, and the exit dose became nearer to that expected for unit density tissue. In most cases it was possible to explain the observed exit doses, but the magnitude of the variation could not have been assessed from radiographs and clinical examination. It is therefore not possible to apply general correction factors, without exit dose measurement (or quantitative tomography) on each patient (Holodny, Gian, Ragazani, Bronstein and Laughlin, 1964; Fedoruk and Johns, 1957). Lithium fluoride powder makes it possible to make a large number of measurements on patients during routine treatment. Such measurements may prove useful for more precise estimates of the dose given in radiotherapeutic procedures, especially where results of lung treatments are being analysed with respect to dose.