leiomyoma of deep soft tissue
Since the time of the 1994 classification, leiomyomas of deep soft tissue have become better recognized and defined and there are now good criteria, based mainly upon mitotic activity and nuclear atypia, for separating these lesions from leiomyosarcoma.
For practical purposes, however, with the exception of retroperitoneal/pelvic lesions occurring in female patients, the presence of any mitotic activity in a deep-seated smooth muscle neoplasm is worrisome for malignancy.
It is also now appreciated that the vast majority of so-called smooth muscle tumours arising in the gastrointestinal tract, mesentery or omentum are in fact better classified as gastrointestinal stromal tumours, most examples of which are characterized by activating mutations of KIT or PDGFRA.
These lesions are described in the volume devoted to tumours of the digestive system.
References
The evolving classification of soft tissue tumours: an update based on the new WHO classification. Fletcher CD. Histopathology. 2006 Jan;48(1):3-12. PMID: #16359532#