Benign uterine leiomyomas (fibroids) are the most common pelvic tumor in women [1-3]. Less commonly, women with a uterine mass presumed to be a leiomyoma are found to have a uterine sarcoma or a leiomyoma variant. The rates of such unanticipated entities have been estimated to be approximately 1:100 for leiomyoma variants and approximately 1:500 for leiomyosarcoma . There are a number of leiomyoma variants in which the smooth muscle tumor manifests one histologic facet of malignancy, yet lacks all others. Other variants are defined by their quasi-malignant capacity to metastasize while remaining histologically and clinically benign. Finally, some smooth muscle tumor variants cannot be definitely classified, and consequently are deemed to have an uncertain malignant potential. (See "Differentiating uterine leiomyomas (fibroids) from uterine sarcomas", section on 'Prevalence of sarcoma after surgery for presumed leiomyomas'.)
Histologically and biologically defined variants of uterine leiomyomas are reviewed here. The diagnosis and management of ordinary uterine leiomyomas, as well as approaches to differentiating leiomyomas from uterine sarcoma, are discussed separately. (See "Epidemiology, clinical manifestations, diagnosis, and natural history of uterine leiomyomas (fibroids)" and "Overview of treatment of uterine leiomyomas (fibroids)" and "Differentiating uterine leiomyomas (fibroids) from uterine sarcomas".)
There are several rare syndromes with leiomyomas or leiomyoma-like lesions that require differentiation from ordinary uterine leiomyomas. The classification of these variants continues to evolve as information about their pathophysiology is gained. The currently used categories are descriptive in nature and are only beginning to be characterized in molecular and genetic terms. Thus, there may be some overlap among some categories.
Definition of benign versus malignant lesions — Leiomyoma variants are classified as benign or malignant based upon histologic features. Gross characteristics may suggest that a lesion is benign or malignant, but do not confirm a diagnosis. While some variants include lesions having extrauterine extension or dissemination, this does not automatically confer a diagnosis of malignancy if the histology is benign. On the other hand, some of the variants have histologic findings that make it difficult to define them as benign or malignant (eg, smooth muscle tumors of uncertain malignant potential). (See 'Smooth muscle tumors of uncertain malignant potential' below.)
Benign smooth muscle tumors (ie, leiomyomas of the usual histologic type or “garden-variety” leiomyomas) are defined as follows :