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

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.

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.

The mean seroma edge extended beyond the clips by 0.3-0.5 cm. In all 120 D(c-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 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).

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.