DOSIMETRIC ANALYSIS OF INTACT BREAST IRRADIATION IN OFF-AXIS PLANES

Purpose/Objective: The purpose of this investigation is to quantify do se inhomogeneity of intact breast irradiation in off-axis planes, and determine how dose inhomogeneity varies according to patient breast si ze and anatomical region of the breast. Methods and Materials: Eleven patients treated with intact breast radiation underwent a treatment-pl anning computer tomography (CT) scan with l-cm slices through the enti re breast. The area of breast tissue was defined on each CT slice. Tre atment planning with lung correction factors was performed using a two -dimensional treatment-planning system that calculates off-axis dose d istributions on a slice-by-slice basis. Each plan utilized tangential beams with matched nondivergent posterior borders and, with collimator rotation to match the chest wall slope. Dose inhomogeneity within the central plane was minimized during treatment planning by the use of a wedge on the lateral tangent field and by the differential weighting of fields. Dose was normalized at the breast and pectoralis major inte rface at midseparation in the central plane. Off-axis dose inhomogenei ty was not considered in the optimization of the treatment plan. Dose distributions were plotted for each I-cm slice, and the area of each i sodose curve within the breast on each CT slice was calculated. The re sults of each slice mere summed to give an approximation of dose-volum e relationships. Results: For the entire population, an average of 10% of the breast volume (range 1-40%) received 110% or greater of the pr escribed dose. Increasing dose inhomogeneity was positively correlated with increasing breast sizes (r = 0.72, p = 0.01-Spearmen rank test). Analysis of dose as a function of location within the breast, reveale d that the greatest dose inhomogeneity occurred in the lower anatomica l quadrants of the breast (p = 0.003-Kruskal-Wallis test). For the gro up, the mean breast volume that received a 110% or greater dose was: 3 0% at 6 cm below central axis, 14% at 4 cm below central axis, 6% at c entral axis, 5% at 4 cm above central axis, and 7% at 6 cm above centr al axis. Conclusion: Our study demonstrates that a significant volume of breast tissue receives 110% or greater of the prescribed dose. This inhomogeneity is greatest in women with larger breast sizes, providin g a possible explanation for the poorer cosmetic result seen in this s ubset of patients compared to women with small breast sizes. In additi on, our results show the greatest dose inhomogeneity in the lower quad rants of the breast. Off-axis dose inhomogeneity should be considered in the planning of tumor bed boosts in women with lower quadrant tumor s. (C) 1997 Elsevier Science Inc.