The dynamic tumor bed: volumetric changes in the lumpectomy cavity during breast-conserving therapy.

PURPOSE: To characterize the magnitude of volume change in the postoperative tumor bed before and during radiotherapy, and to identify any factors associated with large volumetric change.

METHODS AND MATERIALS: Thirty-six consecutive patients with early-stage or preinvasive breast cancer underwent breast-conserving therapy at our institution between June 2006 and October 2007. Computed tomography (CT) scans of the breast were obtained shortly after surgery, before the start of radiotherapy (RT) for treatment planning, and, if applicable, before the tumor bed boost. Postoperative changes, seroma, and surgical clips were used to define the tumor bed through consensus agreement of 3 observers (B.P., D.I., and J.L.). Multiple variables were examined for correlation with volumetric change.

RESULTS: Between the first and last scan obtained (median time, 7.2 weeks), the tumor bed volume decreased at least 20% in 86% of patients (n = 31) and at least 50% in 64% of patients (n = 23). From the postoperative scan to the planning scan (median time, 3 weeks), the tumor bed volume decreased by an average of 49.9%, or approximately 2.1% per postoperative day. From planning scan to boost scan (median interval, 7 weeks), the median tumor bed volume decreased by 44.6%, at an average rate of 0.95% per postoperative day. No single factor was significantly associated with a change in tumor bed volume greater than 20%.

CONCLUSIONS: The average postlumpectomy cavity undergoes dramatic volumetric change after surgery and continues this change during RT. The rate of change is inversely proportional to the duration from surgery. In this study no factors studied predicted large volumetric change.