Tangential breast irradiation: simple improvements.

A series of studies using phantom and in vivo dosimetry measurements with diodes and thermoluminescent dosimetry were undertaken to establish whether simple methods are available to obtain improvements in homogeneity of dose in the treated breast, and reductions of dose to tissues outside it, when using tangential 6MV photon fields. These studies confirmed that the use of an appropriate lung density correction factor in the planning process is likely to cause a reduction in the amount of wedge compensation predicted to be necessary to produce reasonable dose homogeneity in the central axis of the beam. This was shown to be of value in eliminating potential areas of overdosage in the chest wall medial and lateral to the breast mound, and also in reducing unwanted doses to the contralateral breast and lung. Further reductions in dose to contralateral breast were confirmed to occur when the majority, or all, of the wedge compensation predicted necessary is placed on the lateral field and when the lateral tangential field is angled anteriorly in such a way as to align the posterior beam edges. Skin does above, below, and over the breast mound were observed to depend critically upon angle of incidence of the beam, with its consequent effect upon electron build-up, and the position of the breast contour in the beam. Skin doses above and below the breast mound were shown to exceed over the mound itself. This differential effect was observed to increase with increasing wedge compensation. In some situations skin doses below the breast mound nearly reached prescribed dose within the breast. The design and use of simple lead attenuators to reduce these areas of high dosage is discussed.