Can problematic fibroepithelial lesions be accurately classified on core needle biopsies?

Fibroepithelial lesions (FEL) of the breast are notoriously difficult to classify on core needle biopsies. The goal of this study was to evaluate interobserver variability and accuracy of subclassifying difficult FELs into fibroadenoma (FA) and phyllodes tumors (PTs). We identified 50 breast core needle biopsies, initially diagnosed generically as FEL, with subsequent excision and final diagnosis of either FA or benign PT. Five surgical pathologists from one institution independently reviewed these in 3 rounds. The pathologists were blinded to the final excisional diagnosis. Two diagnostic categories were allowed: FA and PT. A set of histologic criteria was provided including the presence of subepithelial condensation, stromal heterogeneity, overgrowth, pleomorphism, fragmentation, cellularity, adipose tissue entrapment, and mitotic count and asked to review the slides for the second round. A third round of interpretations was conducted after each criterion was defined. Interobserver agreement for the diagnosis and each criterion was evaluated using the κ level of agreement. Accuracy of ratings to final diagnosis was calculated using Wilcoxon signed-rank test. κ Values for interobserver agreement were fair for the first and second rounds varying from 0.20 to 0.22, respectively. This increased to 0.27 in round 3. When considering each category, the κ value varied from 0.26 to 0.29 for FA and 0.28 to 0.14 for PT. Overall, there was fair agreement between the pathologists in all categories. The rate of correctly diagnosed cases ranged from 40% in the first round, to 48% in the second round, to 67% in round 3. Overall the pathologists performed better in identifying FA than PT. The accuracy of interpretations was significantly different between the first (40%), second (48%), and third rounds (67%).