A Comparison of Three Dimensional Ultrasound, Clips and CT for Measuring Interfractional Breast Lumpectomy Cavity Motion

Accurate definition of the lumpectomy cavity (LC) is essential for both partial breast and electron boost planning and delivery. The goal of this study is to evaluate CT, 3DUS and clips in the daily target localization of the LC. Twenty whole breast radiation patients, including 10 with clips, underwent two sets of treatment planning CTs (CT1, CT2) with co-registered 3DUS (US1, US2), separated by approximately 6 weeks. The cavities were independently outlined on each CT and 3DUS dataset. Of the 20 patients, 18 had visible cavities. The shifts in cavity position were calculated from CT, 3DUS and clips. The PTV margins required to be added to the CT1 cavity volume in order to fully encompass the CT2 cavity were calculated without and with shifts according to 3DUS or clips. The median cavity displacement ranged from 0.0-3.0 mm with a maximum of up to 17.9 mm. The cavity displacements were not significantly different when calculated based on 3DUS, CT, or clips (p > 0.05). The average PTV margin required to encompass the CT2 cavity was 6.3 mm without shifts, 4.3 mm with 3DUS shifts and 5.2 mm with clip-based shifts. The differences in the margin between no shift and 3DUS or clip-guided shift were found to be significant, while differences between 3DUS and clip-based shifts were not (p > 0.7). With no shift, the mean CT2 volume that lied outside the CT1 was 3.2 cc compared to 2.4 cc (p < 0.03) with 3DUS and 2.8 cc (p < 0.05) with clip-based shift. The volume difference between the shifting methods was not significant (p > 0.9). Without adjusting for the changes in the cavity shape and location, a portion of the volume will be undertreated unless the margins are increased. 3DUS may offer an easily implemented solution to localizing the LC without requiring additional ionizing radiation.