A strategy that simplifies these conundrums and anticipates he future problems of diverging beam junctions, gaps or overlaps is as follows. A common set up point for the current and any future fields is placed in the center of the pelvis approximately 1-2cm superior to a line through the edge of the acetabulum. The example in Figure 12 assumes the rectum, vagina, urethra, or penis should be excluded and that the medial pubis and sichium are not involved. The collimator is rotated so that the Y collimators are oriented axially and over travel is employed. The superior X collimator is set to 0.
Field blocking can further shape the field as needed.
The cross indicates the central ray and can serve as a set up point for future fields as shown in Figure 13. The most challenging situation arises when a third course of treatment must be given such as to sites described in Figure 14. If the same 3 point set up was used for all 3 port areas and beam edges were programmed to be coplanar with previous fields, then there will not be an overlap or gap in the treatment anywhere in the pelvis. Large Pelvic Fields Followed by a Smaller Volume Boost Rectal, endometrial, prostate, bladder, and cervical cancers are often treated by administering a dose to the entire pelvis, followed by continued treatment to a smaller volume to a higher dose. Commonly, the boost volume is in the inferior part of the original large field.