Answer: Sclerosing stromal tumor

Histology: The sections show a solid mass with a peudolobular pattern in which pauci-cellular collagenous and edematous connetive tissue separates cellular nodules. Prominent thin-walled vessels, resembling those of a hemangiopericytoma, are present within the nodules. The cellular component consists of a disorganized mixture of spindle cells and round cells. The round cells have round nuclei and eosinophilic or vacuolated cytoplasm. Immunostains demonstrate that the tumor is strongly positive for progesterone receptor, very focally positive for inhibin and calretinin, essentially negative for estrogen receptor, and negative for CD10.

Discussion: Sclerosing stromal tumor (SST) is a rare ovarian stromal neoplasm. Since its first description in 1973, less than 100 cases have been reported in English literature. Unlike other ovarian stromal tumors, such as fibroma, thecoma and steroid cell tumors, SST tends to occur in young women in the second or third decades of life. While most SSTs are hormonally nonfunctional, a small percentage of patients with SSTs present with symptoms related to excess androgen or estrogen production. Typically, the tumor is recognized as unilateral, well-demarcated ovarian mass, measuring 5-15 cm in diameter. Sections of the tumor are usually solid and white in color, but may show cystic degeneration or focal yellow discoloration. Histologically, it is nodular or pseudonodular, with foci of sclerosis and prominent thin-walled vascular proliferation resembling those of a hemangiopericytoma. Two distinct cell types are characteristic of SSTs: round cells with abundant eosinophilic or vacuolated cytoplasm, and spindle cells resemble fibroblasts. Occasionally, lipid-laden cells are present, morphologically resembling signet ring cells in Krukenberg tumors. The differentials diagnosis include thecoma, fibroma and steroid cell tumor. Immunohistochemically, SSTs are positive for vimentin, and negative for desmin. Inhibin and calretinin expression are often focal. Estrogen and progesterone receptor expression is variable. All SSTs reported so far have been benign. Therefore, conservative management is recommended in patients in the reproductive age.