Image-guided radiotherapy for breast cancer patients: surgical clips as surrogate for breast excision cavity.

PURPOSE: To determine the use of surgical clips as a surrogate for localization of the excision cavity and to quantify the stability of the clips' positions during the course of external beam radiotherapy for breast cancer patients, using cone beam computed tomography (CBCT) scans. METHODS AND MATERIALS: Twenty-one breast cancer patients with surgical clips placed in the breast excision cavity were treated in a supine position with 28 daily fractions. CBCT scans were regularly acquired for a setup correction protocol. Retrospectively, the CBCT scans were registered to the planning CT scans, using gray-value registration of the excision cavity region and chamfer matching of the clips. Subsequently, residual setup errors (systematic [Σ] and random [σ]) of the excision cavity were estimated relative to the clips' registration. Finally, the stability of the clips' positions were quantified as the movement of each separate clip according to the center of gravity of the excision cavity.
RESULTS: When clips were used for online setup corrections, the residual errors of the excision cavity were Σ(left-right) = 1.2, σ(left-right) = 1.0; Σ(cranial-caudal)  = 1.3, σ(cranial-caudal) = 1.2; and Σ(anterior-posterior)  = 0.7, σ(anterior-posterior) = 0.9 mm. Furthermore, the average distance (over all patients) between the clips and centers of gravity of the excision cavities was 18.8 mm (on the planning CT) and was reduced to 17.4 mm (measured on the last CBCT scan).
CONCLUSION: Clips move in the direction of the center of gravity of the excision cavity, on average, 1.4 mm. The clips are good surrogates for locating the excision cavity and providing small residual errors.