A new isocenter shift method for ideal geometric matching of two adjacent fields

The most commonly used technique for matching two adjoining parallel opposed fields has been geometric matching with a skin gap calculation. Many modifications have been suggested in order to minimize the shortcomings of this technique. Most of these, however, are more complex and time consuming in clinical practice, with little dosimetric improvement. We propose a simple new method for adjacent field abutment that climinates hot and cold spots due to different beam divergences. This method is based upon the use of a single match plane for both the cephalad and caudad parallel opposed beams. The divergence angle of these opposed beams is forced to be equal at the junction through the use of the same field length and the same target axis distance (TAD). For the isocentric treatment case, the procedure is summarized as follows. After setting up the cephalad APPA fields to midplane in the conventional manner, the patient support couch is shifted longitudinally in the cephalad direction by a precise distance equal to the cephalad field length. The superior collimator of the caudad field is kept the same as for the cephalad field to ensure equal divergence angles. The actual irradiated volume of the caudad field is adjusted as needed by a large block or an asymmetric jaw. An analogous procedure is described for the nonisocentric case. This new isocenter shift method culminates in several distinct advantages over the other geometric methods. It provides the best homogeneous physical dose distribution in the junctional area (film dosimetry shows no hot or cold spots); it saves substantial time and efforts of dosimetricsts and reduces simulation time since there is no need to do gap calculations (sloping surface and difference in patient separation do not affect the results); and it can be easily verified by radiographic techniques using either skin wires or vertebral bodies as a guide. The isocenter shift method is more accurate and easier to set up than previously reported techniques, with marked dosimetric improvement. Where asymmetric jaws are available, this method should be considered as standard practice.