Abstract
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 (CTVLC) 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 (CTsim) 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 (CTsim to CBCT1) and during RT (CBCT1 to CBCT5). The mean RoC of D for all patients before starting RT was −0.25%/day (range, −1.3 to 1.4) and for WBI patients during RT was −0.15%/day (range, −0.45 to 0.40). Stratified by median CTVLC, the RoC before RT for large CTVLC group (≥25.7cc) was 15 times higher (−0.47%/day) than for small CTVLC group (<25.7 cc) (−0.03%/day), p=0.06. For patients undergoing CTsim < 42 days from surgery, the RoC before RT was −0.43%/day compared to −0.07%/day for patients undergoing CTsim≥42 days from surgery, p=0.12. 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 CTVLC.PACS number: 87.55.de
Highlights
210 Truong et al.: Cone-beam computed tomography (CBCT) IGRT to evaluate lumpectomy cavity variation irradiation (PBI), as randomized data have shown improved local control with the addition of a boost.[1]
Surgical clips placed during breast conserving surgery can aid visualization of the lumpectomy cavity (LC) on radiographs and Computed tomography (CT) to help define the limits of the LC and improve the accuracy of defining the clinical target volumes for partial breast irradiation (PBI) and for the breast boost treatment in whole breast irradiation (WBI).[4,5,6] Variations in the LC during RT for WBI and PBI are not routinely integrated into the planning process and treatment for the majority of patients undergoing breast radiotherapy
During the time interval from CT simulation (CTsim) to CBCT1, LC size decreased in 17 patients (65%)
Summary
210 Truong et al.: CBCT IGRT to evaluate lumpectomy cavity variation irradiation (PBI), as randomized data have shown improved local control with the addition of a boost.[1] Accurate identification of the LC clinical target volume (CTVLC) is essential in identifying breast tissue at highest risk for harboring microscopic disease to ensure tumor control while sparing the normal breast tissue.[2,3] Computed tomography (CT) planning for PBI and WBI has become a standard practice, defining the surgical cavity using CT alone can pose difficulties in its differentiation from normal breast tissue as the breast density varies among patients. CBCT can help determine the optimal timing for simulation of LC boost treatment and PBI so that the CTVLC drawn at simulation most accurately defines real-time LC during RT. The purpose of this study was to evaluate changes in the LC by investigating the rate of change (RoC) in size of the LC (with the distance (D) between the surgical clips and their centroid as a LC size surrogate) as a function of LC volume defined at CT simulation (CTVLC) and time interval between surgery and simulation CT (CTsim)
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