Two Subtypes of Atypical Leiomyoma: Clinical, Histologic, and Molecular Analysis.

Atypical leiomyoma (ALM) is a rare variant of uterine smooth muscle tumors. Several recent studies have suggested that ALM has distinct, but also heterogenous, histologic and molecular features, yet little is known about the biology and histogenesis of ALM. Some have even postulated whether the atypical histologic features represent true atypia or simply degenerative changes. In this study, we analyzed the cytologic features of 60 ALM cases and found that ALM could be further divided into 2 subtypes, type I and type II, based primarily on nuclear features. Type I ALM showed round or oval nuclei, distinct and smooth nuclear membranes, prominent nucleoli with perinucleolar halos, and open coarse chromatin. Type II ALM showed elongated or spindled nuclei, irregular nuclear membranes, pinpoint or no nucleoli, and dark smudgy chromatin. There were also architectural differences between type I and type II ALM. Type I ALM often showed diffuse atypia within the tumor, whereas the atypia in type II ALM was patchy, surrounded by usual-type leiomyoma. The 2 subtypes also differed when we compared the immunohistochemical and molecular patterns. Type II tumors showed significantly higher rates of immunoreactivity for p16, p53, and HMGA2 and showed MED12 mutations more frequently than the type I counterparts. Our findings suggest that the type I and type II subtypes of ALM may arise from 2 different pathways. Type I tumors may be related to fumarate hydratase mutations, whereas type II ALM appear to arise in a existing usual-type leiomyomas.