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Serous Borderline Tumor : Micropapillary

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Micropapillary variant is an aggressive subtype of serous borderline tumor (SBT) which shows micropapillary and/or cribriform architecture and a non-hierarchical branching pattern. It may present as a pure micropapillary tumor or coexist with usual SBT. When present with usual SBT, the micropapillary focus must be at least 5 mm in largest dimension.

The WHO 2014 Classification of ovarian tumors considers micropapillary variant of SBT equivalent to non-invasive low-grade serous carcinoma; however, caution must be exercised in the use of this term as some clinicians may not be aware of this relationship.

The micropapillae are fine, slender, finger-like projections that arise directly from larger papillary cores. They are at least 5 times longer than they are wide. They have scanty or no stromal cores and are lined by cuboidal or polygonal, non-ciliated cells with high N:C ratios and small mildly atypical nuclei. The mitotic index is low but higher than that seen in usual SBT. Some cases show cribriform architecture at the surface of the larger papillae.

Gene expression profile studies suggest that the micropapillary variant is closer to low-grade serous carcinoma than it is to usual SBT and may represent an intermediate step in progression to low-grade serous carcinomas.

Prognosis: Micropapillary variant is more aggressive than usual SBT and presents at advanced stages more frequently (27% of micropapillary cases vs 13% of usual SBT cases). In addition, among the patients with extra-ovarian disease, 50% of cases of micropapillary variant are associated with invasive implants as compared to 8% of cases with usual SBT.

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