Clinical case
A 32-year-old G1P0 at 9 weeks and 3 days gestational age by last menstrual period presents with new onset vaginal bleeding. A transvaginal ultrasound shows no embryo, but notes a complex, echogenic intrauterine mass containing many small cystic spaces ( Fig. 21.1 ). A serum human chorionic gonadotropin (hCG) level is 190,000 IU/L. On physical exam, her chest is clear to auscultation. Speculum exam shows a slightly dilated cervical os with prune-colored bleeding, but no lesions visible on the cervix or in the vagina. Bimanual examination reveals an 11-week size uterus with bilateral adnexal fullness. She undergoes an uneventful ultrasound-guided electric vacuum aspiration of the uterus. The pathology is consistent with complete hydatidiform mole (CHM). A week later, her hCG level is 200,000 IU/L, and the week after that her hCG has climbed to 350,000 IU/L. Chest X-ray shows no metastatic disease. A repeat ultrasound shows a 6 cm heterogeneous and cystic mass with myometrial invasion. The patient desires fertility sparing treatment for her disease. How do you treat this patient?
Epidemiology
Incidence and mortality
Gestational trophoblastic disease (GTD) refers to a category of lesions characterized by abnormal proliferation of placental trophoblasts. Benign forms of GTD include exaggerated placental site, atypical placental site nodule (PSN), and partial and CHM. Malignant forms of GTD are referred to as gestational trophoblastic neoplasia (GTN). GTN includes invasive mole, choriocarcinoma, and the very rare placental site trophoblastic tumor (PSTT) and epithelioid trophoblastic tumor (ETT) (see Section “Pathology” below). Unique among gynecologic cancers, GTN does not require a definitive histopathologic diagnosis. The diagnosis can also be made with laboratory tests alone when there is a persistent elevation of hCG after evacuation of a molar pregnancy.
The precise incidence of GTD is uncertain because data regarding molar pregnancies and trophoblastic neoplasms are not available in most countries. The incidence is thought to be higher in Asia and South America than Western Europe or North America. The most reliable data come from the United Kingdom, were for the past 50 years all GTD cases have been required to undergo registration at one of three national reference centers. From 2000 to 2009, the overall incidence of molar pregnancies in the UK was 1 case per 607 conceptions.
GTN may occur after any gestational event. The histology of postmolar GTN may be invasive mole, choriocarcinoma, PSTT, or ETT. GTN after a non-molar pregnancy is usually choriocarcinoma but may also present as either PSTT or ETT. Approximately 50% of cases of GTN arise from molar pregnancy, 25% from miscarriages or tubal pregnancy, and 25% from term or preterm pregnancy. About 15%–20% of patients after CHM will develop GTN, and 5% will develop metastatic disease. GTN only occurs in 1%–4% of patients after partial hydatidiform mole (PHM) and is rarely metastatic. The incidence of GTN after abortion is estimated at 1 in 15,000 pregnancies, while the incidence after a term pregnancy is estimated at 1 per 150,000 pregnancies. The overall incidence of GTN following all types of pregnancies is estimated at 1 in 40,000 pregnancies.
The International Federation of Gynaecology and Obstetrics (FIGO) and the World Health Organization (WHO) have adopted a prognostic scoring system to risk-stratify patients with GTN ( Table 21.1 ).
| International Federation of Gynaecology and Obstetrics (FIGO)/World Health Organization (WHO) score | 0 | 1 | 2 | 4 |
|---|---|---|---|---|
| Age (years) | < 40 | ≥ 40 | – | – |
| Antecedent pregnancy | Mole | Abortion | Term | – |
| Interval months from index pregnancy | < 4 | 4–6 | 7–13 | > 13 |
| Pretreatment hCG (IU/L) | < 1000 | 1000–10,000 | 10,000–100,000 | > 100,000 |
| Largest tumor mass (cm) | < 3 | 3–5 | > 5 | – |
| Site of metastases | Lung | Spleen, Kidney | Gastro-intestinal | Liver, brain |
| Number of metastases | – | 1–4 | 5–8 | > 8 |
| Previous failed chemotherapy | – | – | Monotherapy | Combined therapy |
Patients with a risk score < 7 are considered “low risk” while patients with a risk score ≥ 7 are considered “high-risk.” Patients with PSTT or ETT are automatically considered high-risk. Risk scores of ≥ 12 are frequently referred to as “ultra high-risk,” although this term has not been universally adopted. The “risk” refers to the likelihood of response to single-agent chemotherapy, but also conveys the risk of mortality. Patients with low-risk disease are first treated with single-agent therapy, usually either methotrexate or actinomycin D, while patients with high-risk disease receive multiagent etoposide-based regimens (more details below). While resistance to first-line chemotherapy in low-risk disease ranges from 20% to 50%, the salvage rate is extremely high, such that overall survival approaches 100%. For high-risk disease, overall survival ranges from 90% to 95%, while in ultra high-risk disease overall survival is 60%–70%. In addition to the FIGO/WHO risk score, patients with a histologic diagnosis of choriocarcinoma or metastatic disease to the brain or liver are at substantially elevated risk of disease-related death. Underscoring the importance of expert consultation for these diseases, the risk of death is also increased by as much as 28-fold for patients with high-risk disease initiating therapy outside an experienced treatment center.
Etiology and risk factors
Molar pregnancies occur due to abnormalities of fertilization causing an excess of paternal chromosomes. CHM are generally diploid (rarely tetraploid), purely androgenetic conceptions, with most (∼ 85%) arising from fertilization of an egg lacking maternal chromosomes by a single sperm that then duplicates its chromosomes (homozygous/monospermic 46,XX). A small subset arises via fertilization by 2 sperm (heterozygous/dispermic 46,XY or 46,XX). PHM are triploid with a single set of maternal and two sets of paternal chromosomes. In both CHM and PHM, mitochondrial DNA remains maternal. The relative imbalance of parental chromosomes is thought to cause an imprinting defect. Genomic imprinting describes an epigenetic phenomenon wherein a gene is expressed only on the maternally or paternally inherited chromosome. Imprinting is a form of gene inactivation. Although a cell may have two copies of a gene, only the copy on the maternal or paternal chromosome is expressed. A classic example is Angelman Syndrome and Prader–Willi Syndrome. Both disorders stem from imprinting on the same region of chromosome 15, but loss of the maternal contribution results in Angelman syndrome while the loss of the paternal contribution leads to Prader–Willi Syndrome. The hypothesis of molar pregnancy as an imprinting defect has been supported by rare cases of biparental moles wherein genome-wide DNA methylation loss in the oocyte produces familial recurrent molar pregnancy.
The cause of nonmolar GTN has not been defined but is also likely epigenetic. While changes such as copy number variation, loss of heterozygosity, and abnormal methylation are seen in gestational choriocarcinomas, molecular studies have been striking for the relative lack of common driver mutations. Even less is known about the causes of PSTT or ETT. However, small studies have suggested that the tumors do share similar transcriptomic profiles.
The leading risk factor for GTD and GTN is maternal age. The risk of complete molar pregnancy ranges from < 1:1000 conceptions for women for ages 18–40, to 1:156 for women aged 45 and above and 1:8 for those aged 50 and above. There is also about a 7-fold increased risk of CHM for adolescents. Risks of PHM do not vary appreciably with maternal age. Interestingly, despite having higher rates of CHM than adults, adolescents have lower rates of postmolar GTN. The risks of both postmolar and nonmolar GTN also increase with advancing maternal age.
Patients with a prior history of GTD are at elevated risk of GTD in subsequent pregnancies. After a molar pregnancy, 1%–2% of patients will have GTD in a later gestation. This risk rises to about 10% after two previous molar pregnancies. Patients with more than two complete molar pregnancies should be referred for genetic testing for possible familial recurrent hydatidiform mole (FRHM).
FRHM describes a family of exceptionally rare hereditary autosomal recessive disorders with incomplete penetrance characterized by the near-inability of affected women to have a noncomplete molar pregnancy. Affected families have multiple members with multiple complete molar pregnancies. The most commonly affected gene is NLRP7 , an important immune response activator that responds to microbial acetylated lipopeptides. Other genes implicated in FRHM include KHDC3L , MEI1 , and c11ORF80 . Unlike sporadic complete moles, which are entirely androgenetic, familial complete moles display chromosomes from both maternal and paternal origins. Women with FRHM can only conceive using an egg donor. While homozygosity is required for the full manifestation of FRHM, carriers have an increased risk of recurrent miscarriage, partial moles, and stillbirth. Penetrance is considered incomplete because rarely normal pregnancies do occur among homozygous carriers.
Pathology
Gestational trophoblastic disease
As described above GTD is defined by abnormal proliferation of trophoblastic tissue and encompasses a spectrum of diseases that can be broadly divided into non-neoplastic and neoplastic diseases. The non-neoplastic diseases include PHM and CHM, in addition to exaggerated implantation site and PSN. The neoplastic diseases, also referred to as GTN, are comprised of choriocarcinoma, PSTT and ETT. Invasive hydatidiform moles are considered a form of GTN because they invade the myometrium and can be found outside of the uterus. However, they are not composed of a clonal proliferation and thus, in the true sense of the word, are not neoplastic.
Although the pathogenesis of the various forms of GTD is not completely understood, recent studies have increased our understanding of some of the entities and their relationship to normal placental trophoblast. There are three types of placental trophoblast: syncytiotrophoblast, cytotrophoblast, and intermediate trophoblast. Syncytiotrophoblast are large, multinucleated, differentiated cells that produce most of the placental hormones. They are located on the surface of the chorionic villi, overlying the cytotrophoblast, and are terminally differentiated cells. Unlike syncytiotrophoblast, cytotrophoblast are smaller, mononuclear cells that have stem cell–like properties and are mitotically active. They form a continuous layer lining the surfaces of the villi. Intermediate trophoblast are large, mononuclear cells that can be further subdivided into three subtypes called implantation site, chorionic type, and villous type intermediate trophoblast. As will be discussed below, these subtypes of intermediate trophoblast give rise to different types of GTD. In addition, immunohistochemical and molecular tests are currently being used as ancillary studies to improve diagnostic accuracy and, thus, patient management.
Given the complexity of the lesions not all of the entities can be discussed in detail in this chapter. Only those entities directly related to the case discussed in this chapter will be presented in detail.
Partial hydatidiform mole
Gross
The gross appearance shows an admixture of enlarged villi and normal appearing villi. The hydropic changes are not so prominent as in complete moles, hence may not be evident grossly.
Microscopic findings
PHM show two villous populations, one that is normal appearing and the other with changes including hydropic change, irregular villous contours, circumferential trophoblastic proliferation, and trophoblastic pseudo-inclusions. Evidence of fetal development is demonstrated by the presence of nucleated red blood cells in the villous capillaries. When the histologic features of PHM are not well-developed they can be easily missed. If there is histologic suspicion, then ancillary testing such as DNA flow cytometry may be performed.
Complete hydatidiform mole
Gross appearance
A well-developed CHM consists of markedly enlarged, edematous chorionic villi that have a translucent appearance. The abnormal villi may vary in size but virtually all of the villi are affected. However, given the identification of abnormal gestations early in the first trimester by ultrasonography, the villi may only demonstrate mild enlargement that may not be readily apparent on gross examination.
Microscopic features
CHM is an abnormal placenta with markedly hydropic chorionic villi with associated trophoblast proliferation due to a purely androgenetic conception. Most CHM (approximately 85%) are due to fertilization of an empty egg by one sperm (monospermic) with reduplication of the genome and are usually diploid (46, XX), however fertilization by 2 sperm (dispermic) can occur and also are usually diploid (46, XX or 46, XY). Due to the purely androgenetic nature of the conceptus there is a lack of embryonic development.
As mentioned above, the chorionic villi of a CHM are enlarged, bulbous and edematous (hydropic) with central cisterns (acellular, empty space) and circumferential proliferation of cytotrophoblast, syncytiotrophoblast, and intermediate villous trophoblast ( Fig. 21.2A and B ). Although vessels are present in the villous stroma they lack the presence of nucleated red blood cells given the absence of fetal development. These features are well developed in CHM evacuated in the second trimester; however, they can be subtle in the first trimester and are referred to as “early CHM.” Consequently, early CHM present diagnostic difficulties especially in the absence of clinical history. The villi do show abnormalities with bulbous, irregularly shaped villi with mild circumferential trophoblast proliferation and stroma with increased cellularity with a myxoid appearance and small vessels with scattered karyorrhectic debris, closely resembling hydropic villi. However, the utilization of immunohistochemistry for p57 is very helpful in recognizing this entity (as discussed below).
Invasive hydatidiform mole
Gross appearance
An invasive mole is detected grossly by the presence of enlarged, irregular chorionic villi invading the myometrium.
Microscopic features
An invasive mole has similar features to a CHM however, the molar villi invade the myometrium and/or uterine vessels. In addition, invasive moles can spread to extrauterine sites including lung, vagina, vulva, and ovary/broad ligament, which is why they are considered clinically malignant despite not actually being neoplastic. In the extrauterine sites, the molar villi are largely confined to vascular structures without invasion of surrounding tissue.
Choriocarcinoma
Gross appearance
The size of choriocarcinoma varies significantly from microscopic foci to very large tumors with extensive hemorrhage and necrosis. When they are grossly apparent they are usually dark red in color and have irregular surfaces. The metastatic lesions are often well-circumscribed with associated hemorrhage.
Microscopic features
This discussion will be limited to gestational choriocarcinoma which are composed of neoplastic syncytiotrophoblast, cytotrophoblast and intermediate villous trophoblast ( Fig. 21.3 ) with morphological features of primitive trophoblast of the previllous stage of placental development. As such, it is considered the most primitive form of GTN. The tumor cells demonstrate marked cytologic atypia and a high mitotic index and usually show areas of necrosis, hemorrhage, and lymphovascular invasion. Historically, the presence of chorionic villi has precluded a diagnosis of choriocarcinoma; however, it has been recently acknowledged that they can occur in the setting of complete/invasive hydatidiform moles. Of note, this diagnosis should be made with caution since trophoblast proliferation is often exuberant in CHM mimicking choriocarcinoma. Furthermore, intraplacental choriocarcinoma is a well-recognized entity in term placentas and can be mistaken for a placental infarct.
Placental site trophoblastic tumor
Gross appearance
The majority of the tumors are well-circumscribed and can either be situated in the myometrium or project into the endometrial cavity. Upon sectioning the tumors are tan and soft usually with only focal areas of necrosis or hemorrhage.
Microscopic features
This tumor is thought to arise from transformed cytotrophoblast that differentiate to resemble implantation type intermediate trophoblast. It is composed largely of mononuclear cells with ample amphophilic cytoplasm that display considerable nuclear pleomorphism with occasional, scattered multinucleated cells ( Fig. 21.4A and B ). The tumor cells form sheets that infiltrate the myometrium by separating the smooth muscle bundles and invade vessels often replacing the vessel walls with a fibrinoid deposit similar to what is seen in the normal implantation site. The mitotic index is usually low (approximately 2–4 mitoses/10 hpf). Although the tumor may cause little tissue destruction, some tumors can show considerable necrosis which is a feature that is associated with malignant behavior.
Epithelioid trophoblastic tumor
Gross appearance
Although they can be located in the uterine corpus a substantial number (approximately 50%) are found in the lower uterine segment or endocervix. They can also occur in extrauterine sites. The tumors range in size from 0.5 to 4.0 cm. The tumors form discrete nodules that can be solid or cystic with areas of hemorrhage and necrosis.
Microscopic features
The tumor nodules are composed of medium-sized, mononucleate trophoblastic cells that are fairly uniform and grow in solid masses, nests or cords and have finely granular eosinophilic to clear cytoplasm with round nuclei and moderate atypia. The tumor cells are associated with an eosinophilic, fibrillary, hyaline-like substance that can be admixed between the cells or in the center of tumor nests. The tumor commonly demonstrates extensive necrosis which can have a geographic appearance ( Fig. 21.5A and B ). The presence of calcification is a unique feature of ETT which is not seen in other GTDs. When the tumor involves the cervix the tumors cells can extend into the cervical epithelium and may be mistaken for a high-grade squamous intraepithelial lesion.
Differential diagnosis
The main differential for CHM is PHM and hydropic villi from nonmolar conceptuses. This is mainly an issue with early CHM as the features characteristic of CHM are not clearly developed.
A p57 immunostain can help in these distinctions as the cytotrophoblast and villous stromal cells of PHM and hydropic villi express p57, while it is lacking in CHM. p57 is a protein encoded by the gene CDKN1C that is paternally imprinted and maternally expressed such that expression is lacking in villi resulting from a completely androgenetic conceptus, the feature that defines CHM. This has become a very powerful tool for identifying early CHM and separating them from PHM and hypdropic villi from nonmolar conceptuses with which the microscopic features have considerable overlap. DNA flow cytometry may be used to distinguish early CHM from PHM as the former are diploid while the latter are triploid. In addition, CHM with detached islands of an admixture or cyto- and syncytiotrophoblast can be hard to distinguish from early choriocarcinomas arising in the setting of a CHM. The diagnosis of choriocarcinoma in this setting should be made with caution as this is an uncommon occurrence. In fact, some pathologists don’t diagnose choriocarcinoma in the presence of villi.
Given that there can be overlapping morphologic features between choriocarcinomas and both PSTT and ETT, it is important to make these distinctions as the treatment regimens may vary. These distinctions are largely made on morphologic grounds however, immunohistochemical stains can be helpful (see below).
ETT and PSTT must be distinguished from PSN and exaggerated implantation site, respectively, that are currently considered their nonneoplastic, benign counterparts, as discussed below.
Immunohistochemistry and differential diagnosis
As mentioned earlier molecular studies have not only contributed to our understanding of GTD but have also resulted in ancillary studies that have improved diagnostic accuracy. The expression of specific proteins in the different types of trophoblast along with determination of proliferative activity has had a profound impact on the differential diagnosis of GTD.
The first step is to determine that it is a trophoblastic lesion. Trophoblast are positive for low molecular weight keratins and a protein, HSD3B1, that is involved in progesterone biosynthesis in the placenta and this combination of markers in this setting is indicative of trophoblast differentiation. Immunostains for distinguishing CHM from PHM and hydropic villi were discussed above. The main algorithm used for the differential for GTD (ETT, PSTT and choriocarcinoma) includes several antibodies ( Fig. 21.6 ).
ETT can mimic PSTT, and PSN. ETT is strongly diffusely positive for p63 while only focally positive for hPL and Mel-CAM, whereas PSTT is p63 negative and shows diffuse strong staining with the other two markers. ETT can also mimic squamous cell carcinoma (SCC). Both tumors are positive for p63; however, cervical SCC does not express trophoblastic markers, and is positive for HR HPV, while the opposite is true of ETT.
Like ETT, PSN expresses intermediate trophoblast markers hence, these markers cannot be used in this differential. However, Ki-67 immunostain is extremely useful in separating the two entities, as its expression is typically low (below 10%) in PSN but much higher in ETT (10%–25%). Additionally, cyclin E is positive in ETT while negative in PSN.
Choriocarcinomas are positive for keratin markers such as AE1/AE3. The syncytiotrophoblastic cells are positive for hCG, HSD3B, and inhibin, while the cytotrophoblasts are positive for GATA3 and SALL4. Of note, syncytiotrophoblasts may be seen in some somatic carcinomas; therefore, a diagnosis of choriocarcinoma should not be made if there is only focal hCG staining in the absence of typical histologic findings.
Molecular findings
Very little is known about the underlying molecular pathogenesis of GTD however, genotyping the lesions using short tandem repeats (STRs) is useful in a number of settings in determining the diagnosis of GTDs, when it is not clear from the histopathologic and immunohistochemical findings. Although a complete discussion of the technique is beyond the scope of this chapter a few instances in which it is applied will be presented. STRs are highly polymorphic DNA sequences that can be used to identify maternal and paternal derived alleles. Thus, the analysis of STRs is very useful in distinguishing CHM, PHM and hydropic villi, as CHM are entirely androgenetic and lack maternally derived alleles, unlike PHM and hydropic villi both of which contain maternal alleles. In addition, STRs can be used to distinguish gestational choriocarcinoma from somatic choriocarcinoma as the later will have a biallelic pattern instead of unique parental alleles as seen in gestational choriocarcinomas.
Diagnosis and workup
Signs and symptoms
Classically, molar pregnancy was associated with vaginal bleeding, excessive uterine enlargement, theca lutein cysts, and early onset hyperemesis gravidarum and preeclampsia. However, this presentation reflects the second trimester manifestations of GTD. In locations where hCG measurement and ultrasound are readily available, most molar pregnancies are now diagnosed in the first trimester (see Section Imaging studies below). Importantly, earlier identification and evacuation of molar pregnancies has not changed the rate of postmolar GTN. For example, at the New England Trophoblastic Disease Center, the average gestational age at diagnosis decreased from 16.5 weeks in the 1960s to 9 weeks in the 2000s. However, the rate of postmolar GTN did not change. This strongly suggests that malignant transformation is an inherent biologic property of certain molar pregnancies and not a trait acquired during disease progression. In both classic descriptions and more modern experience, the most common symptom for molar GTD remains abnormal uterine bleeding. Excessive uterine size, hyperemesis, and anemia may still be seen, but are less common in the first trimester. GTD should be in the differential for any woman presenting with bleeding in early pregnancy.
In cases of nonmolar GTN, abnormal uterine bleeding is also the most common symptom. Postterm choriocarcinoma is in the differential diagnosis for postpartum hemorrhage, particularly delayed postpartum hemorrhage, and should be the leading consideration in any postpartum patient presenting with vaginal bleeding and signs of metastatic disease. Women may alternatively present with amenorrhea. This is particularly true for PSTT or ETT.
In both molar and nonmolar GTN, patients may also be symptomatic from metastatic disease. This may present as hemoptysis, respiratory insufficiency, hemoperitoneum, or vaginal lesions.
Physical exam findings
While second-trimester complete molar pregnancies may present with uterine size greater than dates and palpable adnexal masses on bimanual exam, most patients with first-trimester molar pregnancies will not have notable physical exam findings. However, evaluation of any patient with suspected GTD should include a thorough physical exam to exclude metastatic disease. A speculum examination is essential to exclude cervical or vaginal metastases, which can present as pigmented, fleshly lesions on the cervical portio, vaginal walls, or external genitalia ( Fig. 21.7 ). The exam should also include a thorough auscultation of the lungs to evaluate for pulmonary edema or effusions and a basic neurologic exam to exclude focal neurologic deficits. Rarely, metastases may present as pigmented lesions on the skin, oral mucosa, retina, or nail beds. These may be confused for melanoma, underscoring the importance of obtaining an hCG during the diagnostic evaluation of any malignancy of unknown origin.
