The low power appearance of a serous borderline tumor (SBT) of the ovary consists of multiple papillary structures with varying degrees of complexity lining the cyst wall. The papillary structures show a hierarchical branching pattern from large to progressively smaller papillae and ultimately leading to smaller detached tufts of epithelial cells in cleft like spaces. The papillae are lined by ciliated pseudostratified cuboidal or columnar epithelium (similar to that lining the fallopian tubes). Hobnail cells may be present. The cytologic atypia is minimal with moderate nuclear enlargement and occasional prominent nucleoli. The papillary areas may merge with endometrioid or mucinous foci. The distinction between serous borderline and malignant tumors of the ovary is done solely on the basis of invasion, regardless of whether or not there are peritoneal deposits or metastases to the lymph nodes and distant sites. The presence of microinvasive foci is acceptable within the specturm of SBT. Microinvasion: About 10% of SBT cases show small foci (< 5mm) of microinvasion consisting of clusters of cells in the stroma just beneath the surface component. These tumor cells have abundant eosinophilic cytoplasm like the surface papillae. They don't express ER/PR, have low Ki-67 labeling index and don't worsen the prognosis significantly.