Comparison of the Homogeneity of Breast Dose Distributions With and Without the Medial Wedge

Abstract

Radiation of the intact breast often requires medial and lateral wedges to improve dose homogeneity of its pyramidal shape and to achieve acceptable cosmesis. There is some concern that radiation scatter from the medial wedge may contribute to cancer in the uninvolved breast, yet treatment without the medial wedge is associated with inhomogeneity of magnitudes that affect cosmesis. These homogeneities are identified on treatment plans generated at the central axis (CAX). It is not known if comparing isodose curves at the central axis reflect homogeneity in superior and inferior planes. A study was undertaken to both examine inhomogeneity with and without the medial wedge, and to determine if plan selection at the CAX was representative of homogeneity above and below the CAX. Ten consecutive patients with early breast cancers had cranial, CAX, and caudal CT images of each breast compared with two wedging conditions, lateral only (LW) and medial and lateral wedged conditions (dual wedges = DW). Dosimetry was optimized at the CAX for DW and LW conditions. Dose distributions and hot spots relative to prescribed dose were compared for cranial, CAX, and caudal images. Mean chest wall separations were measured. Six of ten patients had equivalent LW and DW distributions at the levels examined. Only one of these patients had a single off-axis hot spot > 20%. Six patients had comparable LW and DW dosimetry and acceptable hot spots at the central axis, as well as chest wall separations ≤ 22 cm. In conclusion, if isodose configurations are commensurate at the CAX, these patients will have homogeneity above and below the CAX. In patients with chest wall separations ≤ 22 cm, treatment without the medial wedge is feasible, sparing the contralateral breast dose with little compromise to inhomogeneity in the treated breast.