Answer: Exaggerated placental site (EPS)

Histology: The section shows atypical mononuclear and multinucleated cells infiltrating the endomyometrium. The cells are large, with smudgy and hyperchromatic nuclei, but no mitotic activity is evident. The infiltrate lacks a confluent growth pattern, and there is no necrosis. Villi are present.

Discussion: This is a case of exaggerated placental site (EPS). It is frequently associated with complete hydatidiform mole but occurs in approximately 1.6 % of normal pregnancies and abortions from the first trimester. Histologically, this lesion is characterized by an extensive infiltration of endometrium and myometrium by implantation site intermediate trophoblasts. The smooth muscle cells of the myometrium are separated by the cords, nests, and individual intermediate trophoblasts. Cytologiclly, the trophoblasts are similar to their counterparts in normal implantation sites. They contain abundant eosinophilic cytoplasm and hyperchromatic and irregular nuclei. Many cells are multinucleated. Despite that, the implantation site is not disrupted. Necrosis is also absent. Mitotic activity is not increased. The Ki-67 proliferation is zero or near zero in the intermediate trophoblats. The Ki-67 immunohistochemical stain highlights only the background lymphocytes. The trophoblasts in exaggerated placental site retain the immunophenotype of normal imtermediate trophoblasts. Those cells are strongly positive for Mel-CAM (CD146) and hPL. The most important and difficult differential diagnosis is from placental site trophobastic tumor. Even though the latter displays confluent masses of trophoblastic cells, unequivocal mitotic figures, and absence of chorionic villi, the two entities pose similar morphologic features of exuberant myometrial infiltration by intermediate trophoblasts that bear identical immunoprofiles. Occasionally, EPS may be mistaken for symplastic leimyoma due to the presence of intermediate trophoblasts resembling atypical smooth muscle cells. However, the presence of villi, the infiltrative growth pattern and immunopositivity to Mel-CAM and hPL will confirm the diagnosis of EPS. Biologically, exaggerated placental site represents the extreme end of normal implantation. It is not a true neoplastic lesion and always resolves spontaneously or after curettage. It dose not appear to carry an increased risk of persistent gestational trophoblastic disease.