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Placental Site Trophoblastic Tumor (PSTT) is a rare malignant trophoblastic tumor arising from extravillous intermediate trophoblasts at the placental implantation site.

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It is part of the Gestational trophoblastic neopolasia (GTN) spectrum but is distinct from choriocarcinoma and ETT in terms of pathology, clinical behavior, hormone production, and treatment response.

Epidemiology

Accounts for <2% of GTN
Most often occurs in women of reproductive age
Typically arises months to years (mean ~2 years) after any gestational event (term pregnancy, miscarriage, or molar pregnancy)

Pathology

Feature	Description
Cell Origin	Extravillous (intermediate) trophoblasts
Histology	Sheets of monomorphic cells infiltrating myometrium
Villi	Absent
Invasion	Deep myometrial and vascular invasion common
Immunohistochemistry	Positive for hPL, Mel-CAM (CD146), and cytokeratin; weak β-hCG

Clinical Features

Irregular uterine bleeding (common presenting symptom)
Amenorrhea or postpartum bleeding
Uterine mass
β-hCG is typically low or only mildly elevated (<2500 IU/L)
Possible metastases to lung, vagina, lymph nodes, or peritoneum in advanced stages

Laboratory Markers:

Marker	Typical Finding
β-hCG	Low or modest elevation
hPL (human placental lactogen)	Strong and diffuse positivity
Ki-67	Lower proliferation index (10–30%)