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Localization was performed using an Elekta Synergy: a linear accelerator with conebeam CT (CBCT) and fluoroscopic capability, with in-house software for clip detection and correction calculation. The procedure was performed weekly on whole breast RT patients with clips. Patients were scanned in treatment position and corrected by couch translation. A verify scan was then acquired before proceeding to treat. Fluoroscopy was used to assess breathing motion. The whole process: scan, correct, and verify, typically added 15 minutes to treatment time. To separate patient vs. system sources of residual error, this procedure was also performed on solid water phantoms with clips. Setup margin assessment was based on two factors: 1) efficacy of the correction calculation (i.e., how good are clips at representing the cavity) and 2) accuracy in making the calculated correction (residuals). Data for 2) was acquired in the aforementioned scans. Data for 1) came from the earlier study, from which we selected a subset of 12 patients (8 APBI) with scan dates consistent with the RTOG 0413 calendar. Cavity shrinkage was also assessed for these twelve cases. Finally, the effect of adding a rotation correction was studied retrospectively in 23 patients.
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