Experimental Determination of Absorbed Dose from X-Rays Near the Interface of Soft Tissue and Other Material

Abstract

When cells of soft tissue adjacent to bone, such as osteocytes or marrow cells, are irradiated by x rays of less than 250 kv, the absorbed dose is substantially higher than that in soft tissue remote from bone for the same exposure dose. A similar effect occurs when tissue culture cells are irradiated on glass. Experimental measurements of the ratio of absorbed dose to exposure dose near a soft tissue-bone interface and a tissue-glass interface are made. The method employed involves measurement of ionization in a gaseous layer using a parallel-plate ionization chamber. One electrode of thls chamber is made of muscle equivalent plastic, the other of bone equivalent plastic or glass. One electrode is movable so that the spacing can be varied. The use of either muscle equivalent gas or bone equivalent gas allows determination of the dose rate at very small distances away from the interface in either muscle or bene. The dose rate as a function of distance from the interface is obtained from the slope of the curve of ionization current vs. electrode spacing. Distribution curves of rad/r in the region of interest (less than 50 microns) about a bone-muscle interface are given for x-ray potentialsmore » of 36, 50, 140, and 210 kv. For the same four x-ray beams, curves of average rad/r in a soft tissue layer between two bone layers are given as a function of tissue thickness up to 25 microns. For the tissue-glass interface, curves of the relative increase in absorbed dose in muscle attributed to glass backing vs. tissue thickness are shown for x-ray tube potentials of 50, sults with calculated values and biological studies of other workers are made where possible.« less