Twin Gestation with Complete Hydatidiform Mole

John D MacKenzie, MD - Case Coordinator
Aaron D Sodickson, MD, PhD - Radiology Discussion
Miguel N Rivera, MD - Pathology Discussion
Frank S David, MD, PhD - Pathology Contributor
Mary C Frates, MD - Attending Radiologist
Pablo R Ros, MD, MPH - Attending Radiologist

April 22, 2002

Presentation

Imaging Findings

On sagittal ultrasound (US) images, the placenta is visible anteriorly. Posteriorly, an area of mixed echogenicity is seen. There are echogenic areas in close proximity to the placenta, though it is not clear whether of echogenic material is arising from the placenta or the fetus, or whether it is completely separate. The fetal survey (not shown) was normal.

On magnetic resonance (MR) images, portions of a normal fetus are visible. The mass appears to be separate from the fetus and adjacent to the placenta. The images do not clearly delineate the relationship between the mass and the umbilical cord. There is no hyperintensity on either T1 and T2-weighted images to suggest hemorrhage.

Differential Diagnosis

Discussion

The woman carried the pregnancy to term, and then a total hysterectomy was performed. The first gross image illustrates the inner surface of the uterus, with the cervix on the right side. In the second image, a perpendicular cut into the placenta shows a second, abnormal placenta with large, hydropic chorionic villi. The first microscopic image shows the interface between the two placentas. The normal structure has small villi. The abnormal structure has very large villi with large amounts of trophoblast on their surface. The diagnosis of complete mole was made using a new test based on p57, a paternally imprinted antigen. In a complete mole (with paternal chromosomes only), both copies of the gene are silent. In normal and partial molar pregnancy, there is a maternal copy of the gene which is expressed. In this case, the normal villi stained positive for p57 (brown color), while abnormal villi did not. This confirms the diagnosis of Complete Hydatidiform mole. Gross pathology images courtesy of Frank S David, MD, PhD.

Radiology Discussion

Gestational trophoblastic disease (GTD) describes a spectrum of disorders that can develop in subsequent pregnancy, whether previous pregnancies were normal or abnormal. Hydatidiform mole is the most common manifestation, representing 85% of cases. Hydatidiform mole is noninvasive and remains confined to the endometrium. Choriadenoma destruens is a locally invasive (myometrium) manifestation that represents 13% of cases of GTD. Two percent of cases are described as choriocarcinoma, which is locally invasive (myometrium and parametrium). Choriocarcinoma is also highly malignant, spreading hematogenously to the lungs, brain, liver, kidneys, bones, and GI tract. These masses are often discovered when ßhCG levels remain high 8-10 weeks following evacuation of a molar pregnancy. All three entities produce abnormally high levels of ßhCG. The rarest form of GTD, placental type trophoblastic tumor, is malignant in 15-20% of cases. It is unique in that it causes a rise in human placental lactogen (hPL) levels, and low hCG levels. Some form of GTD develops in 0.05-0.1% of pregnancies (1 in 1000-2000) in the US; it is more common in Latin America and Asia.

Hydatidiform mole is characterized by edematous placenta, the proliferation of trophoblasts, and the presence of swollen, hydropic chorionic villi. The most common clinical signs include hyperemesis (high ßhCG), vaginal bleeding, large-for-dates uterus, and the passage of swollen vesicular tissue per the vagina. Preeclamsia and pregnancy-induced hypertension are common complications.

Hydatidiform mole has, in the past, been subdivided into "complete" and "partial" manifestations. Complete hydatidiform mole involves the entire placenta and generally begins with the fertilization of an "empty egg"—no maternal genetic material is involved. There is no identifiable fetal tissue—the cells of the placenta are typically diploid, with paternal 46XX being more common than 46XY. As in the case of this patient, it is possible to have a dichorionic gestation with one normal fetus and a coexistent complete mole. Hydatidiform mole does have malignant potential.

The term "partial mole" is a misnomer—the etiology is completely different from that of a hydatidiform mole. Triploid pregnancy is a much more accurate term. Such cases arise when 2 sperm fertilize a single egg, resulting in a 69XXX triploid fetus. When this occurs, the placenta becomes hydropic and the fetus is highly dysmorphic. Triploid pregnancy is uniformly fatal to the fetus and has no malignant potential.

The radiographic appearance of hydatidiform mole is highly variable, depending mainly on the size of the chorionic villi. The villi may range from 1-2 mm up to 3 cm in size. On US imaging, the mass is typically a uniformly hyperechoic soft tissue mass filling the entire endometrial cavity. Numerous tiny cysts may also be present, contributing (with the hydropic villi) to a "snowstorm" appearance. On MR images, the cystic mass often has high signal on T2-weighted images. In 50% of hydatidiform mole cases, Theca Lutein cysts (large, multi-septated ovarian cysts) will develop bilaterally. These are thought to result from the high levels of ßhCG.

The differential diagnosis for the typical radiographic appearance of hydatidiform mole includes:

• Hydropic degeneration of placenta: enlarged chorionic villi, usually associated with missed/ incomplete abortion (normal ßhCG levels)
• Degenerated uterine leiomyoma
• Incomplete abortion – retained products of conception, with hemorrhage
• Choriocarcinoma
• Uterine dysgerminoma, sarcoma, lymphoma

References

Kurtz AB, Middleton WD. Ultrasound: the requisites. Mosby-Year Book, Inc.; 1996.

Brant WE, Helms CA. Fundamental of diagnostic imaging. 2nd ed. Lippincott,Williams & Wilkins; 1999

Weissleder R, Rieumont MJ, Wittenberg J. Primer of diagnostic imaging. 2nd ed. Mosby; 1997

Dahnert W. Dahnert’s radiology review manual. 3rd ed. Williams & Wilkins; 1998

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