Morphology and DNA content analysis in the evaluation of first trimester placentas for partial hydatidiform mole (PHM)

Partial hydatidiform moles (PHM) have defined villous abnormalities and are usually triploid. Their diagnosis often can be made by morphology alone, without confirmation of ploidy, but considerable interobserver variability exists. Other genetic abnormalities such as trisomy can result in placentas with abnormal villous morphology (AVM) similar to that seen in PHMs, leading to diagnostic confusion. Twenty-three cases originally diagnosed as either PHM or AVM were independently reviewed by 3 pathologists. The consensus diagnosis was PHM in 14 cases and AVM in 9. Cases with AVM showed insufficient features for an unequivocal diagnosis of PHM. DNA content was determined on paraffin-embedded tissue by fluorescence in situ hybridization (FISH) using a chromosome 1 centromeric probe and by image cytometry (IC). Thirteen of 14 cases (93%) classified as PHM were triploid by both FISH and IC. Seven cases of AVM were diploid by FISH and IC, and 1 was triploid by FISH and IC. One of the 9 cases of AVM was determined to be trisomy 18 by karyotyping. This good correlation of consensus diagnosis with ploidy data was much greater than that obtained based on original diagnoses. Comparative genomic hybridization performed on 6 cases of AVM showed gain of chromosome 21 in 1 case and loss of X in another. PHMs displayed at least 3 of the following histologic features: 2 discrete populations of villi, circumferential mild trophoblastic hyperplasia, trophoblastic inclusions, prominent scalloping of villi, cistern formation. Nontriploid AVMs displayed at most 2 of the diagnostic features of PHM. Placentas with genetic abnormalities other than triploidy can display morphologic changes suggestive of PHM and can be misinterpreted as such by routine light microscopy. Stringent application of morphologic criteria improves the correlation of the diagnosis of PHM with triploidy. Copyright (C) 2000 by W.B. Saunders Company.