Effect of Lumpectomy Cavity Volume Change on the Clinical Target Volume for Accelerated Partial Breast Irradiation: A Deformable Registration Study

Purpose: Previous studies have shown that lumpectomy cavity volumes can change significantly in the weeks following surgery. The effect of this volume change on the surrounding tissue that constitutes the clinical target volume (CTV) for accelerated partial breast irradiation and boost treatment after whole breast irradiation has not been previously studied. In the present study, we used deformable registration to estimate the effect of lumpectomy cavity volume changes on the CTV for accelerated partial breast irradiation and discuss the implications for target construction. Methods and Materials: The data from 13 accelerated partial breast irradiation patients were retrospectively analyzed. Deformable registration was used to propagate contours from the initial planning computed tomography scan to a later computed tomography scan acquired at the start of treatment. The changes in cavity volume and CTV, distance between cavity and CTV contours (i.e., CTV margin), and CTV localization error after cavity registration were determined. Results: The mean ± standard deviation change in cavity volume and CTV between the two computed tomography scans was -35% ± 23% and -14% ± 12%, respectively. An increase in the cavity-to-CTV margin of 2 ± 2 mm was required to encompass the CTV, and this increase correlated with the cavity volume change. Because changes in the cavity and CTV were not identical, a localization error of 2-3 mm in the CTV center of mass occurred when the cavity was used as the reference for image guidance. Conclusion: Deformable registration suggested that CTV margins do not remain constant as the cavity volume changes. This finding has implications for planning target volume and CTV construction.