Using segmented fields to treat the breast

One standard method for treating intact breast uses wedges for missing tissue compensation and shapes the tangential fields to follow the chest wall and spare lung tissue. The limitations of this technique are that it produces areas of increased dose cranially and caudally where the breast tissue thins, and it is hard to use in combination with MLC in those cases where the wedge fixes the direction of leaf motion. This paper presents a simple IMRT technique that uses field segments within the standard tangential fields to replace the physical or dynamic wedge. Defining the segment shapes is easily accomplished by finding a cut-plane at the isocenter and perpendicular to one of the tangential fields. Selected isodose curves are projected onto this plane, printed, and reentered through the digitizer as additional field segments. The approximate weighting is determined by the size of the dose steps and checked by observing the shape of the isodose lines on a transverse plane. Correct weighting gives an approximately straight isodose line running through the isocenter and perpendicular to the beam center axis. The process is repeated for the other tangent field and all segments are added. Results showed that the regions of increased dose (superiorly and inferiorly) were decreased relative to the standard planning method when just two segments are added to each tangential field. Additionally, the MLC is freed to rotate to provide an optimum match to the segment boundaries for this shrinking field, IMRT technique.