30 yo patient, circumscribed uterine mass #Gynpath 
Placental site trophoblastic tumor!
* Derived from implantation site intermediate trophoblast
* Polygonal cells with irregular, hyperchromatic nuclei
* Dense eosinophilic cytoplasm
* "Splitting" of myometrial fibers
* Tumor cells often invade into vessels (lower right)
* Derived from implantation site intermediate trophoblast
* Polygonal cells with irregular, hyperchromatic nuclei
* Dense eosinophilic cytoplasm
* "Splitting" of myometrial fibers
* Tumor cells often invade into vessels (lower right)
* Can be self-limited, but some cases are agressive, depends on stage, age, and duration from antecedent pregnancy.
Agree with comments! This lesion does have a wide differential diagnosis, including other trophoblastic lesions and mesenchymal lesions, here are a few:
* Exaggerated implantation site: the "other" lesion of implantation type intermediate trophoblast. Multinucleated cells more common here, proliferative activity is lower than PSTT, can be admixed with hyalinized material and chorionic villi
* Epithelioid Trophoblastic Tumor: Lesion of chorionic tpye intermediate trophoblast, with smaller, more rounded cells, necrosis, calcifications, also p63 positive (PSTT is neg)
*Choriocarcinoma: Biphasic pattern with syncytiotrophoblast giant cells
*Smooth muscle tumors, especially epithelioid variants (conventional leiomyosarcoma pictured here), but PSTT is muscle marker negative.
