Figure 15 shows a generic pelvic field that has been designed with the isocenter shifted inferiorly near the center of the prostate, uterus, or rectum boost volume that will be treated later in a higher dose. The boost field is shown in Figure 16 and in this instance, is centered on the prostate. The technique is illustrated with the patient supine but is equally appropriate for a prone patient. The superior border of the breast tangent field is established and an axial match plane perpendicular to the floor created. The posterior plane of the field is non-diverging and the collimator is programmed so that the toe half of the wedge is blocked for both the medial and lateral tangents.
Treatment is given with the non-diverging heel half of the beam.
A cerrobend block defines the superior end of the field along the x-axis match plane. In practice, many women exceed that length and the tangential beams are centered in the middle of the breast and the couch is angled 5-8 degrees depending upon the field length, collimator, and gantry angles. It would be possible to give a field within a field treatment. The large field could receive 2.0Gy, and the boost oblique field an additional 1.0Gy. A simulation film showing the asymmetric nature of the field is shown in Fig.