Answer: Pregnancy luteoma

Histology: This lesion shows a mass of cells with abundant eosinophilic cytoplasm, central nuclei, some which are hyperchromatic, and prominent nucleoli. In areas there are follicle-like spaces lined by thin endothelial cells.

Discussion: Pregnancy luteomas (PL) are often incidental findings during Caesarean sections, although they have been detected during routine pelvic examinations, in young women. These lesions can range in size and have been reported to reach up to 20 cm in dimension. On cut section they are solid and fleshy and have a brown-red appearance. This case demonstrates the typical histologic findings of these entities. Of note, these lesions may contain frequent and sometimes atypical mitotic figures. Lesions that are removed post-partum may also show degenerated cells with pale cytoplasm, pyknotic nuclei, and an infiltration by lymphocytes and fibrosis. Unlike luteinized thecomas, PL are often multiple, contain little or no lipid and lack a background of fibroma or typical thecoma. Both PL and juvenile granulosa cell tumors contain follicle-like spaces and both are reactive with immunohistochemical stains for inhibin. To make matters more difficult, both juvenile granulosa cell tumors and the PL have been associated with virilization. However, unlike the granulosa cell tumor, PL is frequently multiple, bilateral, or both. PL has been mistaken for metastatic malignant melanomas. The lipochrome pigment in PL can be mistaken for melanin, and bilaterality and multinodularity also supports the diagnosis of metastatic melanoma. However, immunohistochemical stains showing positivity for inhibin, and negative staining with HMB-45 and S-100, can help facilitate the diagnosis of PL. Ultimately, it should be emphasized that any ovarian mass composed entirely of lipid-free steroid type cells from a pregnant woman in the third trimester should be considered to be a pregnancy luteoma unless there is evidence to the contrary.