A Novel Radio-Opaque Hydrogel Marker Can Be Used to Decrease Interobserver Variability in the Delineation of the Lumpectomy Cavity

Abstract

Accurate lumpectomy cavity (LC) delineation is necessary to define a target for cavity boost or partial breast irradiation (PBI), but the LC is not always well visualized on CT. Surgical clips mark discrete points along the cavity margin, leaving the majority of the LC border undefined. We introduced a novel radio-opaque hydrogel marker to lumpectomy cavities of female cadavers to assess the effect on inter-observer variability in defining the lumpectomy cavity during CT-based treatment planning. Three refrigerated, unfixed female cadaveric specimens underwent lumpectomy procedures with oncoplastic closures by breast surgeons. Saline was introduced via an indwelling catheter to eliminate air. Baseline simulation CT images were performed, and the saline was replaced by the hydrogel marker via the catheter. The hydrogel marker used in this cadaveric series is an FDA-approved injectable polyethylene glycol-based hydrogel designed to be visible under CT, MR and ultrasound imaging for several months after implantation. LC contouring was performed on the axial slices of the simulation CT data sets by three breast radiation oncologists. To assess change in visibility of the LC with the addition of hydrogel, a cavity visualization score (CVS) of 1-5 was assigned to each LC by each physician observer, and the median CVS was compared for each cavity with and without the hydrogel. LC volume variability was assessed by Vmax/Vmin and the coefficient of variation for volume (COVvol), defined as the standard deviation of the mean divided by the mean. A conformity index (CI) was used to evaluate variability of the LC shape, defined as the sum of the intersections of all possible pairs of delineations divided by the sum of unions of all possible pairs. Paired-samples (repeated measures) t tests were used to compare differences between groups. The median CVS increased in 7/9 cavities with the hydrogel. For all cavities, the median change in CVS when hydrogel was introduced was +2 (range, 0 to +2). The mean LC volume decreased from 17.8 cm3 to 7.0 cm3 with the addition of the hydrogel (p = 0.002). Mean COVvol decreased from 0.41 to 0.28 (p = 0.025) and Vmax/Vmin decreased from 2.6 to 1.8 (p = 0.023). The mean CI increased from 0.30 to 0.47 (p = 0.009). The addition of a radio-opaque hydrogel marker to lumpectomy cavities with oncoplastic closure at the time of surgery may increase cavity visualization and decrease inter-observer variability of volume and shape when delineating the cavity for adjuvant radiation therapy. In addition, there is a significant decrease in the delineated LC volume, which could potentially have dosimetric benefits. Further study is warranted.