Placental site trophoblastic tumor

Placental site trophoblastic tumour (PSTT) is rare and one of the least common (~ 0.2% ⁷) forms of the gestational trophoblastic disease (GTD).

Epidemiology

PSTT typically occurs in women of reproductive age with the average age around 30. It may occur after a normal pregnancy, molar pregnancy or even after a terminated pregnancy. Time to the presentation can widely vary between 1 week and 14 years.

Clinical presentation

Presenting symptoms can include either abnormal vaginal bleeding and amenorrhoea ⁶. Up to 30% of patients may present with metastases at the time of diagnosis ¹⁰. 

Pathology

PSTT is composed of neoplastic transformation of intermediate trophoblastic cells that form a polypoid mass that may be within the endometrial canal or the myometrium.

It may be divided into two main types depending on vascularity ⁴:

• hypervascular
• relatively hypovascular

Markers

• in contrast to other forms of the gestational trophoblastic disease, PSTT produces small amounts of ß-human chorionic gonadotropin; this is thought to be due to relative lack of syncytiotrophoblastic tissue within a tumour
• human placental lactogen (hPL) may be increased

Radiographic features

There can be some imaging overlap with an invasive mole and choriocarcinoma. One of the key roles of imaging is to attempt to clarify the vascularity of a tumour.

Pelvic ultrasound

Transvaginal ultrasonographic features of PSTT are classified into three types depending on the location (within the uterine cavity or the myometrium) and the characteristics (solid or cystic tissue) of the lesion: 

• Typetype 1: heterogeneous solid mass located in the uterine cavity, with minimal to a moderate degree of vascularizationvascularisation on colorcolour Doppler imaging. 
• Typetype 2: heterogeneousheterogeneous solid mass within the myometrium (invasion ≥50% of its thickness), with minimal to a high degree of vascularizationvascularisation on colorcolour Doppler imaging. 
• Typetype 3: cystic lesions within the myometrium, with a high degree of vascularizationvascularisation (lacunar-type lesions), indicating an arteriovenousarterio-venous shunt.

The lesions are of varying sizes with ill-defined borders within the uterus (maximum diameter of the masses is 2–6 cm, with a mean mass diameter of 3 cm).

ColorColour Doppler exploration is performed to evaluate blood flow within the mass (in the centercentre or at the border of tumorstumours) and the degree of vascularizationvascularisation which is subjectively classified as:

• Minimalminimal: <5 points of Doppler flow signals exploration.
• Moderatemoderate: ≥5 points of Doppler flow signals or single branching vessel exploration.
• Highhigh: multiple focal vessels exploration.

Doppler waveform analysis of vessels within the PSTT typically demonstrates high-velocity flow and low impedance.

Pelvic MRI

Placental site trophoblastic tumour may present as a heterogenous endometrial + myometrial mass although appearances on MRI can be non-specific. The junctional zone is disrupted. In the majority of the cases, there are cystic spaces and vascular structures.

Reported signal characteristics include ^1,3:

• T1: typically isointense compared with healthy myometrium
• T2: isointense to slightly hyperintense compared with myometrium

Treatment and prognosis

A hysterectomy is the primary mode of treatment in the majority of cases ⁷. A tumour tends to remain confined to the uterus until late in its course although can eventually metastasise to lung, liver, lymph nodes, and brain. Metastases can occur many years after the initial diagnosis ³.

Their biological behaviour is variable, ranging from benign lesions confined to the uterus, to highly aggressive malignant disease with systemic metastases. The main negative prognostic variables are time to presentation from last known pregnancy and mitotic index.

While there can be a good prognosis with localized disease, the tumours tend to be less sensitive to chemotherapy than other forms of gestational trophoblastic disease ⁶. However, chemotherapy may still play a role with lesions that are not amenable for curative surgery ⁷. 

History and etymology

First described by R J Kurman et.al in 1976 as a "trophoblastic pseudo-tumour". The term PSTT was later coined by R E Scully and R H Young in 1981 ⁵.

Differential diagnosis

For MRI appearances consider

• retained products of conception (RPOC) ²: intracavitary lesion/tissue compared with intramyometrial in PSTT
• other forms of the gestational trophoblastic disease