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BORDERLINE SEROUS TUMOR OF THE OVARY WITH MICROINVASION. The characteristic feature of borderline serous tumor is the hierarchical branching of the papillae, where the papillae progressively branch from larger ones to smaller one and finally into tufts of epithelial cells. The cells show only mild to moderate atypia. A cytadenofibroma may have similar proliferation, but by convention should be less than 5% of the tumor. Focal mucinous change may also occur. Papillary clear cell carcinoma simulating borderline tumor will show a WT1-ve, ER-ve, PR-ve immunoprofile in contrast to the positive profile of the serous tumor. Often clusters of cells with abundant eosinophilic cytoplasm are seen at the surface of papillae (star, image 2; oval image 3) or in lymphatic like spaces either as single cells or small papillary clusters. This theoretically represents microinvasion which is variably defined by different authors (<3 or 5 mm or 10mm2). However, recent evidence suggests that the two patterns of invasion have vastly different prognosis. The AEC (abundant eosinophilic cells) are p16 positive and reflect a terminally differentiated senescent phenotype. If invasion is exclusively composed of this cell type, then these do not portend a poor prognosis.(Am J Surg Pathol 2014;38:743–755). These cells are significantly more often seen in BRAF mutated tumors, and are perhaps associated with a better prognosis. (Am J Surg Pathol 2014;38:1603–1611) (
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