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Online localization of the lumpectomy cavity using clips realistically improves setup accuracy by ~3 mm (vs. no localization). The absolute margin estimate of <5 mm (including setup and breathing) in previous work may have been optimistic because 1) this estimate did not consider residual errors that must arise in clinic; 2) this estimate counted cavity shrinkage as reduced margin. Though shrinkage occurs, even in the timeframe of APBI, it is not clear how this impacts margin design. A more realistic estimate is 6 mm for setup alone. To this would be added a small breathing margin to account for the limited excursion observed. Adding a rotation correction does not appear to allow further margin reduction. However, delaying the planning CT till >4 weeks post-surgery may improve the accuracy of clip-based correction due to slowing of cavity shrinkage. Finally, though our current CBCT technique does not permit direct visualization of the lumpectomy cavity (hence clips), the question, "is conebeam CT capable of cavity-based localization?" has not yet been answered to our knowledge.
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