On-line localization of the lumpectomy cavity using surgical clips

PURPOSE: To present an on-line image guidance procedure for external beam accelerated partial breast irradiation based on cone-beam computed tomography (CBCT) imaging of surgical clips; and to estimate the possible clinical target volume (CTV) to planning target volume (PTV) margin reduction allowed by this technique. METHODS AND MATERIALS: Clips in the CBCT image are detected automatically using in-house software. The treatment couch is translated according to the shift in the clips' center of mass between the planning and CBCT images. Three components for the PTV margin are considered: (1) breathing, (2) surrogate error (i.e., error in cavity position after perfect setup to clips), and (3) residual error (i.e., error arising from the inability to execute a perfect setup to clips due to technological limitations, such as couch travel precision). These factors were input into a standard formula for CTV-to-PTV margin calculation. RESULTS: The average magnitude of clip-based corrections was 7 +/- 2 mm (10 patients, 44 fractions). After localization, the residual error magnitude was 1.6 +/- 1.3 mm, justifying an isotropic CTV-to-PTV margin of approximately 6 mm, including breathing and surrogate error. CONCLUSIONS: On-line localization of the lumpectomy cavity using surgical clips is technically feasible from the standpoint of equipment, time, and process, making possible a decreased CTV-to-PTV margin for accelerated partial breast irradiation. Because the procedure is exclusively target based, additional monitoring of critical structures may be advisable.