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 ( Dc–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 Dc–s s, 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 Dc–smax (The maximal Dc–s of each patient) > 0.5 cm and 7 patients had Dc–smax > 1 cm. With the electron fields, only 46.7% (14/30) had D90 (The minimal dose received by 90% of the planning target volume (PTV)) > 90% and geographical miss (any portion of the PTV receiving 0.5 cm was associated with D90 < 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.