The routine whole pelvic field for radiation therapy is indicated by the long-dashed outline. The upper margin forms along the L5-S1 contact in order to minimize the exposure to loops of small bowel that generally lie superiorly to this area. Bone marrow volumes are also kept to a minimum exposure. Lateral field margins are generally located 1 cm outside the widest point of the true pelvis with diagonals extending from this widest point to the upper end of the sacroiliac joint.
Lower margins are along a line just below the pubic symphysis.
The bifurcation of the aorta and the upper half of the common iliac vessels fall outside of the normal treatment field. There is a price, however, in the form of an increased exposure to small bowel and spinal cord. Berman and others have reported that small bowel adhesions from extended field radiation are less common using this extraperitoneal approach rather than the transperitoneal route. 3 Figure 1. This path is indicated as a bold line in figure 1 and in perspective view in figure 2 .