j09 EFFECTS OF EXPOSURE GEOMETRY 80 J (a) Hllo S100F% 601 = Nab) © . <= BOL) *ON ow molse Ser b) IMME DIATE BOMB| 4 GAMMA ; c) Cont GAM UNI. 4) Co® CROSSFIRE 8) Co®?, BILATERAL | g 607 SL eS | | 2 SOP 4 4 4 # 10 FROM E ACH SIDE C) TOTAL DOSE RECEIVED d) MULTILATERAL OR ROTATIONAL EXPOSURE} (c) 0 60 Pee) wy 90 ( Z Ne) eT Ne 4 4oF 2 30 NO) 2 ao; ~ 30)() = 20} o A w 10 * | on 4 S 20+ C4 5 10 15 20 26 DEPTH (cm) 5 10 15 20 25 DEPTH (cm) 4 E [TTT To) FALLOUT GAMMA 120 Gabji/2 OF TOTAL DOSE w 120 ;— | is i ow (0) 0 80 4 SN“) 607a) CROSSFIRE EXPOSURE | 40 |b) RING, OR 47 EXPOSURE R | 20'- B 5 7 10 5 20 26 DEPTH (cm) Fig. 4. Depth-dose curves for Co® y-radiation in Masonite Iphantom material for several y exposure geometries; depth dose expressed as per cent of entrance air dose. dose deposition results even with highly energetic radiations, and that with this type of ‘total-body” exposure the distal surface may receive only a very small percentage of the “dose” that the phantom or animal, by convention, is said to have received. The marked falloff in dose results both from absorption in the phantom and from the inverse square effect. (By inverse square effect alone, the dose at the distal side, B (Fig. 1), is 63% of the entrance air dose (see reference 2).) Bilateral exposure. In an effort to overcome the marked lack of uniformity of depth dose obtained with unilateral exposure, a number of imvestigators have