Problems in the histological assessment of hydatidiform moles: A study on consensus diagnosis and ploidy status by fluorescent in situ hybridisation

Hydropic villi in products of conception continue to pose a diagnostic problem for the anatomical pathologist. It is important to distinguish between complete hydatidiform mole (CM), partial hydatidiform mole (PM) and hydropic degeneration (HD), as hydatidiform moles (especially CM) have a tendency to develop persistent trophoblastic disease. Several studies have demonstrated interobserver variability in the diagnosis of the three conditions, but there have been no studies testing the accuracy of the consensus diagnosis of pathologists experienced in the field. In this study four anatomical pathologists with experience in diagnosing hydatidiform moles selected five cases of HD, seven cases of PM and ten cases of CM on the basis of consensus diagnosis using established criteria. Ploidy studies were done on these 22 cases using fluorescent in situ hybridisation. The 15 cases of HD and CM were diploid, confirming the histological diagnosis. However only five of the seven cases of PM were triploid, the other two being diploid. Review of these two diploid cases showed a mixture of small and large villi with moderate to marked trophoblastic proliferation. On the basis of the significant trophoblastic proliferation and the DNA information, the two cases were reclassified as early complete moles. This study demonstrates that even pathologists experienced in the field have difficulty separating PM from CM. The findings suggest that, in the absence of DNA information, a lesion with hydropic villi showing moderate to marked trophoblastic proliferation should be classified as a complete mole, even if there is a mixture of small and large villi. Ploidy studies are an important adjunct to histological diagnosis, especially when there is an overlap of features.