Figure 1. Histomorphologic features of micropapillary serous borderline tumor and BRAF-mutation-associated serous borderline tumor.

(A-C) Micropapillary serous borderline tumor demonstrating elongated slender papillae (5X greater in length than width) with non-hierarchical branching (“Medusa-head” pattern). (D-F) Serous borderline tumor with eosinophilic tumor cells, characteristic of the BRAF^V600E mutation. At low-power magnification, the tumor has a crowded, hypercellular appearance, reminiscent of the micropapillary variant. However, closer inspection reveals short, blunt papillae with detached single cells and small cell clusters. These distinctive cells, which are often exfoliated, but also present within the tumor epithelium, exhibit dense/glassy eosinophilic cytoplasm occupying at least 50% the cell area, and at least twice the amount of cytoplasm compared with non-eosinophilic cells in the tumor. (A, D) 2X objective; (B, E) 10X objective; and (C, F) 20X objective.