Absorption of X-Rays by Tissues of the Head and Neck

Abstract

During a series of experiments on dogs involving saliva analyses prior to and following irradiation, it became desirable to know the dose received by the salivary glands, teeth, and jaws following unilateral and bilateral exposures. Pertinent data could not be found in the literature. Experimental The dogs used were from other experiments and were due to be sacrificed. This was accomplished by an overdose of Nembutal intravenously. Three animals were used, weighing 7.2 kg., 13.8 kg., and 25 kg. Immediately after death the following areas of the neck were displayed by blunt dissection, and cavities were made large enough to accommodate a 25-r Victoreen chamber: (a) subcutaneous, lateral to the left parotid; (b) medial to the left parotid; (c) medial to the left submaxillary; (d) medial to the right submaxillary; (e) medial to the right parotid; (f) subcutaneous, lateral to the right parotid. The various areas for positioning of the ionization chamber are shown schematically in Figure 1. All measurements were made along the central ray, at points corresponding to intersection of the vertical lines from Curve A. The radiation factors were as follows: 200 kv.p., filtration 0.25 mm. Cu plus 1.0 mm. Al, h.v.l. 0.8 mm. Cu, target-skin distance 57 cm., cone area 20 × 20 cm. The head of the animal was cut in a transverse sagittal plane, through the salivary glands, so that actual distances between positions, versus doses received, could be plotted for anatomical correlation. The effect of the teeth and mandible upon x-ray absorption was determined by placing the ionization chamber in the various intra-oral positions indicated along the central ray in Figure 2. After completion of the exposures, a transverse section was cut through the lower jaw and the distances were measured as indicated on the abscissa scale. Results and Discussion The transmission curves on the three dogs of different weights and head measurements can be seen in Figure 3. Since these curves are of the same type, the data from the largest dog (total diameter along central ray 13 cm., comparable to human dimensions) was used to compile Figure 1. As can be seen in Figure 1, the rate of absorption is nearly linear until the second submaxillary gland is penetrated; the absorption is then considerably increased. The bilateral exposure curve is determined by the sum of the entrance and exit doses of the two unilateral exposures. These are expressed as per cent of one (unilateral) incident air dose. In unilateral exposure of 100 r, the skin received approximately 120 r; however, in a bilateral exposure of 100 r, the skin receives approximately 138 r and the parotid gland 160 r, or the entrance plus the exit dose. The effect of the mandible and teeth on unilateral and bilateral exposures can be seen in Figure 2. If the unilateral curve is compared with the unilateral curve through the salivary glands (Fig. 1), it can be seen there is more absorption by tissues of the mandible.