Reconsideration of the Diagnostic Criteria for an Atypical Placental Site Nodule Comparing Typical Placental Site Nodule of the Uterus: A Report of Two Cases.

Placental site nodules (PSNs) are non-neoplastic remnants of chorionic-type intermediate trophoblastic cells from a previous gestation that form a well-defined single nodule or multiple nodules in the uterine and extrauterine sites. As the cases of PSNs transformed into gestational trophoblastic tumors were described in the literature, "atypical placental site nodules" (APSNs) have been considered as putative transitional lesions between PSNs and gestational trophoblastic tumors. Although histologic criteria and cutoff point of Ki-67 proliferation index for differentiating an APSN from a typical PSN have not been clearly defined, nodules larger than 5 mm with increased cellularity, a corded or nested appearance, marked nuclear atypia, increased mitotic activity, and an increased Ki-67 proliferation index (>5% or >8%) of intermediate trophoblastic cells seem to be accepted as diagnostic criteria for APSNs. However, some of the criteria, including lesion size and histologic features of the trophoblastic cells in the nodule are not only subjective but have features inherent of the intermediate trophoblastic cells of the fetal membrane and a typical PSN. We thought that it is not reasonable to consider them as diagnostic features of APSNs, if not associated with cellular proliferation. We present 2 cases of incidentally identified PSNs that were larger than 10 mm in size with a corded or nested arrangement of trophoblastic cells, which could have been categorized as APSNs according to the currently proposed criteria to discuss whether the currently proposed diagnostic criteria for APSNs are appropriate.