Planning the breast boost: How accurately do surgical clips represent the CT seroma?

Abstract

Background and purpose To measure the distance between surgical clips and edge of CT-defined seroma in a coronal plane in women who have undergone wide local excision of breast cancer and to evaluate dosimetric coverage of CT-defined boost volumes by conventional clip-based electron fields. Materials and methods Planning CT images of 30 lumpectomy cavities from 30 patients were reviewed. All seroma cavities had at least 4 clips and Cavity Visualization Score ⩾3. Distances between clips and seroma edge ( D c–s) were measured at the radial margins for each patient. Clips-based electron fields were generated by including all the clips with 2 cm margin in the coronal plane and three-dimensional conformal radiotherapy plans (3D-CRT) were devised based on CT tumor beds (CT-TBs). The parameters of dose–volume histogram between the two boost treatment plans were analyzed. Results The mean seroma edge extended beyond the clips by 0.3–0.5 cm. In all 120 D c–ss, 76.7% were ⩽0.5 cm, 8.3% were >1 cm and 15% were between 0.5 and 1 cm. Twenty patients (20/30) had D c–smax (The maximal D c–s of each patient) > 0.5 cm and 7 patients had D c–smax > 1 cm. With the electron fields, only 46.7% (14/30) had D 90 (The minimal dose received by 90% of the planning target volume (PTV)) > 90% and geographical miss (any portion of the PTV receiving <50% of the prescribed dose) was found in 36.7% (11/30). D c–smax > 0.5 cm was associated with D 90 < 90% ( P < 0.001) and >1 cm was associated with geographic miss ( P = 0.001). Conclusions Surgical clips are not always consistent with the edge of seroma. Electron boost field based on clips leads to insufficient dose coverage to the CT-TB. 3D-CRT planning should be considered to ameliorate the dose coverage to the tumor bed.