560 BOND, CRONKITE, SONDHAUS, TMIRIE, ROBERTSON, AND BORG employed the “bilateral exposure’ technique (see the excellent work of Tullis, 11). This procedure is identical to the unilateral exposure, except that one-half of the “total dose” is administered from each side. Thus, if a total of ‘300 r’” is to be given, 150 ras measured free in air at the proximal skin surface is given from side -l (Pig. 1). The remaining 150 r is then administered from side B. The depth-dose pattern for each separate exposure to Co® y-rays and the total obtained by combining the values obtained with each separate exposure are shownin Fig. 4.1. It can be seen from the curve that the tissue dose throughout the phantom is remarkably uniform when contrasted with that obtained with unilateral exposure, and that a maximumvariation of only 7% is obtained in traversing the phantom. Of equal importance, however, is the fact that the lissue dose at no point in the phanfam exceeds 62% of the entrance air dose, the dose that the phantom, by convention, is said to hare received. The reason for this diserepancy lies mainly in the fact tha‘, during each half-exposure, the distal side of the phantom is receiving only a very small percentage of the dose received by the proximal] side, and, on adding. the half-exposures, the total falls far short of the dose said to have been given (see under “crossfire”? exposure below for additional reasons). If the midline air dose, instead of the entrance air dose, is taken as the total exposure, the resulting curve retains the shape noted above, but becomes 70% (instead of 55%) at the midline. This applies to unilateral irradiation as well. Thusit is seen that use of the midline rather than the entrance air dose tends to equalize the tissue dose and the total air dose, but does not accomplish this fully. Mudtilateral exposure. In an effort further to improve the pattern of dose deposition, or in some stated instances to simulate fallout y-radiation, several investigators have utilized more complicated exposure procedures, such as multilateral, rotational, ring, crossfire, or +7 geometries. By multilateral is meant ‘‘total-body” exposure as with bilateral exposure, except that smaller equal fractions of the “total dose” are delivered from more than two “sides.” For example, one-fourth of the total dose is delivered to each of four 90-degree intervals around the body axis. The depth-dose pattern obtained (four equal exposures) is shown as curve ¢, Fig. 4.1. It is seen that this is no improvement over bilateral exposure. The basic difficulty of bilateral exposure is not corrected, since with each fractional exposure, the distal side always receives a very small per cent of the entrance air exposure dose. It can be shown easily that, independent of the numberof exposures carried out in this fashion, and as a result of symmetry, the midline dose remains the same. The dose at A and B (Fig. 1) decreases less than 2% in going frombilateral we met ttt pet a ari to multilateral (any number of exposures) geometry. Similarly, the dose at intermediate points such as C and J) (Fig. 1) changes only by a very few per cent as the numberoffractional exposures is increased.