Hydatidiform mole (HM) disease is a preneoplastic disorder of abnormal placental growth, which may develop into gestational trophoblastic disease including choriocarcinoma. Three genetic origins of HM are recognized. A diandric diploid conceptus results in a complete mole (CM). Partial hydatidiform (PM) typically results from a diandric triploid conceptus.1 Finally, biparental HM is attributable to specific gene mutations.2
Previous studies of the incidence of HM have shown differences in the incidence rates of HM.3,4 These differences may be attributable to a number of factors including geographic regional variability,5 variation in obstetrical practices, and varying histopathologic criteria. Changes in screening, detection, and diagnostic methods may alter disease incidence. Recent developments in obstetrical practice and pathologic diagnosis have the potential to increase detection of HM and have altered the incidence of HM. The routine use of obstetrical ultrasonography in the first trimester of pregnancy has led to early and accurate detection of pregnancy loss and earlier detection of HM, often at an asymptomatic stage,6 albeit with both high false-positive and false-negative detection rates for HM.7 Abnormal ultrasonographic placental findings suspicious for HM can alert the pathologist to undertake a more thorough and detailed pathologic examination of spontaneous and missed abortion specimens—possibly leading to increased detection of HM. The pathologic diagnosis of HM is now often made before the eleventh week of gestation rather than the classical 14 to 16 weeks of gestation if the features that characterize early CM and PM are recognized. However, the histopathologic diagnosis and classification of HM at this early stage are so imprecise that the published epidemiologic data on HM, especially PM, are suspect.8 Although the diagnosis of HM was previously based solely on histopathologic features, pathologic diagnosis can now be improved using immunohistochemistry (IHC) for p57 and molecular genotyping (MG).9–13
Given the variable data on the incidence of HM in previous reports, many of which are now older than a decade, and with these recent developments in current obstetrical and pathologic practice, a reassessment of the incidence of HM is warranted. This institutional-based study was undertaken to re-examine the incidence of HM in modern obstetrical practice using advanced pathologic diagnostic techniques.
Mount Sinai Hospital is a teaching hospital in Toronto and has a major focus in obstetrical care. Obstetrical ultrasonography is almost universally applied in the first trimester of gestation. All pregnancy loss (abortion) specimens from clinics and operating rooms are submitted to the pathology department for examination. Pregnancy loss specimens are submitted for histopathologic examination, and if any features are suspicious for HM, the case is referred to the gynecologic pathology team for assessment. Gynecologic pathology cases are reviewed and signed out by 3 pathologists (S.N., M.C.C., T.J.C.). Diagnosis of CM requires the typical histopathologic appearance for CM alone or atypical villous morphology with absent immunohistochemical villous staining for p57. A diagnosis of PM requires the typical appearance for PM, which is characterized by irregular chorionic villi with geographic outlines, trophoblastic inclusions, some or focal cisternal formation within villi, and mild circumferential trophoblastic hyperplasia. Because of cost constraints, MG analysis is not performed if these classic features of PM are present. Cases with an atypical villous morphology in which the chorionic villi show only some of these features and are equivocal for PM are submitted for adjunctive MG analysis. Cases with atypical villous morphology and a molecular microsatellite genotype of diandric triploidy or triploidy are also designated as PM. Final pathologic diagnosis, then, is based on histopathologic findings and selective use of IHC and MG.
The immunohistochemical analysis for p57 and genotyping methodologies used by the gynecologic pathology service has been described in detail previously.14 In brief, IHC analysis for p57 protein expression (IHC) is performed on slides prepared from paraffin-embedded sections using a concentrated mouse antihuman antibody p57kip2 clone 25B2 (Leica/Novocastra, United Kingdom). Molecular genotyping analysis determining the ploidy and parental contribution is performed on microdissected formalin-fixed paraffin-embedded sections of maternal and conceptus tissues using genomic DNA recovered from microdissected tissues using a Pinpoint Slide DNA Isolation System (Zymo Research, D3001) and subsequently purified using a Zymo Spin I Column. After measurement of the DNA quality and quantity using a NanoDrop spectrophotometer (Thermo Scientific, Wilmingtion, Del), MG analysis of both conceptus and maternal DNA samples is performed using an Aneufast QF-PCR kit (Molgentix, Barcelona, Spain). This assay contains 19 highly polymorphic short tandem repeat markers on chromosomes 13, 18, 21, X, and Y and 2 nonpolymorphic markers for sex determination, combined in 2 multiplex reactions. In this analysis, the conceptus ploidy status of these chromosomes and parental contribution of each short tandem repeat can be determined by comparing the conceptus allelic pattern to the maternal allelic pattern as per the best practice guideline of the Clinical Molecular Genetics Society.15
A retrospective review of consecutive hospital cases of HM from January 1, 2013, to March 31, 2015, a 27-month period, was undertaken. All HM cases submitted for consultation from beyond the hospital were excluded from the review. In each case, the use of IHC, MG analysis, and final diagnosis was recorded. The number of deliveries during this time was identified.
Forty-nine HM cases were identified in the 27-month period consisting of 18 CMs and 31 PMs. (Table 1). The diagnosis of CM was based on histopathology, with IHC confirmation performed in 17 of 18 cases. The diagnosis of PM was based on histopathology with or without p57 IHC in 7 of 31 cases. In the remaining 24 PM cases, histopathology was equivocal and adjunctive MG undertaken; diandric triploidy was identified in 21 cases. In 3 cases of AVM, triploidy was identified and led to a diagnosis of PM. Overall, adjunctive MG was undertaken in approximately one half of HM cases.
Partial mole was more prevalent with the ratio of CM/PM being 1:1.72. During the 27-month period, there were 14,944 deliveries. The incidence of CM and PM was 1.2 and 2.1 per 1000 deliveries, respectively. These incidences correspond to 1 CM per 833 deliveries and 1 PM per 476 deliveries. The aggregate incidence of HM was 3.3 per 1000 deliveries (1 HM per 303 deliveries). If the expected incidence of hydatidiform molar disease in North America between 1.0 and 2.0 per 1000 deliveries is assumed to represent a normally distributed 1-σ confidence interval, our observed HM incidence (3.3/1000) is significantly greater than this expected incidence (P < 0.001).
The cases in our cohort were derived solely from an institution with contemporary obstetrical and ultrasonographic practices, and it is the first study of the incidence of HM using consecutive cases from a single institution with the pathologic diagnosis based on histopathology supplemented by selective immunohistochemical and MG analyses. Just more than half of HM diagnoses (25/49), including all CM cases, were made by histopathology with or without adjunctive IHC testing (Table 1). In the remaining 24 of 49 cases, the morphologic findings of AVM were equivocal for PM. In traditional histopathologic practice, these cases would likely have been labeled as “suspicious for PM” or possibly “nonmolar abortus.”16
The selective use of MG in these cases, however, led to a firm diagnosis of PM. Molecular genotyping of suspected cases of PM yields definitive diagnosis of cases, which previously would have been considered nondiagnostic.
This study identified an incidence of HM of 3.3 per 1000 deliveries (Table 1). For comparison, reviews and text sources often state that the frequency of HM is approximately 1 to 2 per 1000 pregnancies in North America and Europe3,4,17,18 and are based on original reports of HM epidemiology that used a variety of methodologies and reference parameters. Comparison of older studies in particular yielded highly inconsistent findings.19 Table 2 lists original studies from North America and Europe that were conducted after the morphologic features of CM and PM were defined after the elucidation of the cytogenetic origins of PM.1,31 The methodology of these studies varied considerably. With a single exception, all studies used histopathology only to define HM. Central histopathology review was conducted in a minority of studies. Among the published studies, the incidence of HM was reported with reference to pregnancies, deliveries, live births, and “naturally terminating pregnancies,” which makes comparison with our finding challenging. Nevertheless, our study’s incidence of HM (3.3 per 1000 deliveries) is among the highest, if not the highest, identified to date (reliable pregnancy and abortus numbers are not available, so HM pregnancies or abortus ratios cannot be determined). The probable increased incidence rate of HM in this study is likely attributable to the selective use of MG analysis with subsequent increased detection of PM. Obstetrical ultrasonographic findings may also have served to alert pathologists to a suspicion of HM. Using selective ploidy analysis by DNA flow cytometry to detect triploidy, Jeffers et al27 detected 1.9 HM per 1000 pregnancies in a retrospective study. The most recent study from the Charing Cross Hospital Trophoblastic Disease Service registry, which uses histopathologic central review, only reported 2.17 HM per 1000 live births. The most objective and sensitive measure of the true incidence and classification of HM is achieved only when the selective use of MG analysis is incorporated into pathologic assessment of each case suspected of being HM to identify diandric triploid conceptuses, the genetic basis for PM.
Institutional or hospital-based studies do have limitations because they are subject to patient referral bias, which may lead to an overestimate of disease incidence.32–34 However, population-based epidemiologic data for HM are often unavailable. Similar to the practice of other cancer registries,34 the province of Ontario does not collect data of HM. A few central registries for HM do exist29,30 but are based on histopathologic diagnosis only. Consequently, hospital-based studies using advanced diagnostic techniques can still contribute to our understanding of the epidemiology of HM. Hospital-based studies may be subject to confounding factors and biases. Studies of HM are made by researchers in centers who have an interest in the disease. The proportion of pregnancies, which are terminated, and the proportion of pregnancy losses, which are treated medically and not available for pathologic examination, may affect measurement of HM incidence. Ethnicity and women’s age distribution in the cohort may also impact HM incidence. Racial identity is not available in our patient cohort.
Partial mole is often underdiagnosed by obstetricians and pathologists because it can be difficult or impossible to differentiate from nonmolar abortuses.16,21,35 Earlier studies of the epidemiology of HM that relied solely on histopathologic diagnosis of PM and CM concluded that CM was more common than PM with the CM/PM ratio being as high as 4:1 (Table 2). Similarly, a 2006 Danish study using ploidy analysis of 267 histologically confirmed HM found that only a minority (105) were PMs.36 Only half the cases registered in the Charing Cross registry in 1999 were identified as PM16 although PM slightly exceeded CM in a more recent 2010 report.30 In our study, the PM incidence was greater than that of CM, with a CM/PM ratio of 1:1.72. Two other studies, which have used flow cytometric or cytogenetic techniques to identify PM, have demonstrated similar ratios. Jeffers et al27 using selective flow cytometry identified a CM/PM ratio of 1:3 (Table 2). A CM/PM ratio of 1:2 was identified by Matsuura et al21 in a cohort of cases who were subjected to cytogenetic analysis. A third study of products of conception morphologically suspicious for HM used genotyping for definitive diagnosis and identified 2.5 PMs for each CM.9 The cytogenetic and MG evidence from studies that have not relied solely on histopathologic diagnosis suggests that CM is approximately half as common as PM. This ratio of CM/PM could be used as a laboratory benchmark to monitor the histopathologic diagnosis of HM and to assess the performance of HM registries.
Our data suggest that the selective use of IHC and MG analysis improves the sensitivity of HM diagnosis and PM, in particular. Does this increased sensitivity improve clinical outcomes? Because the progression of PM to gestational trophoblastic disease is rare, in the order of 1%, failure to detect a significant proportion of PM may have only a minimal adverse impact on clinical outcomes, at least on the population health level. Rather than to adopt MG into routine practice, it has been suggested that it is more practical and less expensive to follow women with findings “suspicious for HM” with beta human chorionic gonadotropin assessments.37 This management approach, however, commits women to unnecessary monitoring and postponement of reproductive activity. Our alternative approach consisting of selective use of IHC and MG increases the specificity of a diagnosis of HM and identifies women with nonmolar abortus who can avoid any further investigation and follow-up. Our earlier study found that more than a third of cases with a preliminary diagnosis of HM, or suspicious for HM based on histology alone and then referred in pathology consultation, had a nonmolar abortus.14
If the incidence of HM has remained constant in Europe and North America, our data suggest that HM, especially PM, has been underrecognized in the past. By implication, some cases of GTD, which have followed apparently normal pregnancies, may be attributable to undetected HM. Indeed, it has been hypothesized that persistent GTD is rarely, if ever, derived from a nonmolar first-trimester pregnancy loss.38
In summary, at our institution, the incidence of HM was 3.3 HM per 1000 deliveries, which is greater than most studies’ findings. Approximately one half of HM diagnoses were based solely on histopathologic examination with or without IHC. The remaining HM’s had equivocal histopathology, and MG was used to identify PM. The incidence of PM is almost double than that of CM if MG or cytogenetic techniques supplement histopathologic examination. This ratio can be used as a benchmark for laboratory diagnosis and HM registries.
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