EFFECTS OF EXPOSURE GEOMETRY 61 Rotational exposure.® This type of exposure in which the sourceis fixed and the phantom is allowed to rotate on its vertical axis can be regarded as the limiting case of multilateral exposure, and the curve essentially superimposes on that for multilateral exposure (curve d, Fig. 44). Thus, there is no advantage of this type of exposure over the bilateral or multiport geometries. Identical results are obtained if the phantom is held constant and the source is allowed to revolve about the phantom at a constant TSD of 100 em. Crossfire technique. With the crossfire technique, only a single exposure from two opposing “point” sources energized simultancously is used, as opposed to the bilateral technique in which two exposures, first one side and then the other, are made with a single source. The resulting dose pattern is shown as curve a, Fig. “4B. It is apparent that the shape of the curve is negligibly different from that obtained with bilateral, multilateral, or rotational techniques, and that the tissue dose is still considerably below the air exposure dose that the phantomis said to have received. The reason for the lowtissue dose relative to air dose may not be immediately apparent, since with crossfire technique the air dose threughout the exposure volume is essentially constant. It is easily seen, however, if one considers that as soon as the animal or phantom is introduced the ‘skin’ dose at either side (and throughout the phantom) inumediately drops considerably because of absorption in the tissue or phantom. Thus, the entire curve is well below the entrance air dose. The crossfire curve is higher than the bilateral curve because of what might be regarded as an artifact of dosimetry resulting from the manner in which azr dose is measured with the two techniques. This can be seen as follows: With the bilateral technique, the total air “dose”? administered is the sum of two entrance air doses from the two half-exposures. With the crossfire technique, the total air “dose” given is the sum of the entrance air dose from one machine and the exif air dose from the opposite machine (less by inverse square). Thus the total air ‘‘dose’” with crossfire measured at either surface of the exposure volume (1 or B, Fig. 1), is less than with bilateral, and the tissue dose, in terms of the per cent of air ‘‘dose,” is correspondingly greater. It should be noted that exposure with crossfire for one half the fofal time for both Lalf-exposures with bilateral (two tubes on simultaneously with crossfire) yields a tissue-dose curve that superimposes on the bilateral curve. Since, as noted, the air dose for the same total time is less with crossfire, however, the exposure time with crossfire for the same total air ‘dose’? is longer ‘This method of exposure should be clearly differentiated from the multiple-port or rotational exposure used in radiotherapy of tumors. In the elinic, a collimated beamis emploved which at any given time exposes, in theory, only the tumor mass and a small volume of over- Iving skin and tissue at any instant. Thus, with multiple-port or rotational therapy, the deep tumor, always in the field, receives a maximum dose and any given portion of overlying skin receives a minimum dose.