Cone‐beam computed tomography image guided therapy to evaluate lumpectomy cavity variation before and during breast radiotherapy

The purpose of this study was to evaluate the rate of change (RoC) in the size of the lumpectomy cavity (LC) before and during breast radiotherapy (RT) using cone‐beam computed tomography (CBCT), relative to the initial LC volume at CT simulation ([Formula: see text]) and timing from surgery. A prospective institutional review board‐approved study included 26 patients undergoing breast RT: 20 whole breast irradiation (WBI) patients and six partial breast irradiation (PBI) patients, with surgical clips outlining the LC. The patients underwent CT simulation ([Formula: see text]) followed by five CBCTs during RT, once daily for PBI and once weekly for WBI. The distance between surgical clips and their centroid (D) acted as a surrogate for LC size. The RoC of the LC size, defined as the percentage change of D between two scans divided by the time interval in days between the scans, was calculated before ([Formula: see text] to CBCT1) and during RT (CBCT1 to CBCT5). The mean RoC of D for all patients before starting RT was [Formula: see text] (range, [Formula: see text] to 1.4) and for WBI patients during RT was [Formula: see text] (range, [Formula: see text] to 0.40). Stratified by median [Formula: see text] , the RoC before RT for large [Formula: see text] group ([Formula: see text]) was 15 times higher ([Formula: see text]) than for small [Formula: see text] group ([Formula: see text] cc) ([Formula: see text]), [Formula: see text]. For patients undergoing [Formula: see text] [Formula: see text] from surgery, the RoC before RT was [Formula: see text] compared to [Formula: see text] for patients undergoing [Formula: see text] days from surgery, [Formula: see text]. For breast cancer RT, the rate of change of the LC is affected by the initial cavity size and the timing from surgery. Resimulation closer to the time of boost treatment should be considered in patients who are initially simulated within six weeks of surgery and/or with large [Formula: see text]. PACS number: 87.55.de