Case Report: Tubal Atypical Placental Site Nodule.

Placental site nodule (PSN) is a benign proliferation of chorionic-type intermediate trophoblastic cells that forms a tumor-like lesion. Most PSNs are intrauterine, but a few have been reported outside the uterus, including in fallopian tubes. PSN is related to epithelioid trophoblastic tumor (ETT) in that both are composed of chorionic-type intermediate trophoblastic cells, while ETT is hypercellular and contains trophoblastic cells with increased nuclear atypia and a higher Ki-67 proliferation index as compared with PSN. Occasionally, an intermediate stage between a PSN and an ETT is observed, and such a lesion is often recognized as an atypical PSN (aPSN) characterized by trophoblastic cells exhibiting morphologic features in transition from a conventional PSN to an ETT. aPSN has been thought to exhibit benign behavior; however, it has also been reported that up to 15% of aPSN lesions either coexist with, or subsequently develop into, ETT. To the best of our knowledge, there has been no case report of an aPSN in an extrauterine site. Here, we reported a highly unusual case of tubal aPSN, which illustrates several key features associated with PSN and its possible pathogenesis.