Leiomyoma with bizarre nuclei (LM-BN) is a rare variant of leiomyoma with overall benign clinical course. It has histologic features showing focal or diffuse nuclear atypia surrounded by usual type leiomyoma. Uterine leiomyosarcomas (LMS) are a group of rare and aggressive malignancies with limited treatment options available. The potential association between LM-BN with LMS is largely unknown. In this study, we report 2 cases of uterine smooth muscle tumor with typical histologic and molecular evidence of LM-BN, which are associated with its progression to the malignant counterpart of LMS. We summarize the detailed histologic, morphologic, and genomic characteristics of these 2 sets of cases. Our findings suggest that LMS progressing from preexisting LM-BN can be one of the tumor pathogenesis pathways in uterine leiomyosarcomas.

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