Criteria for malignancy in nonvisceral smooth muscle tumors

Reliable prediction of the behavior of smooth muscle tumors is notoriously problematic at many anatomic locations. Relatively few studies have addressed the minimal histologic criteria for malignancy in smooth muscle tumors of soft tissue and the presently available criteria are not only inconsistently applied but also vary at different anatomic sites. It has been widely believed that the presence of any mitotic activity in a deep-seated smooth muscle tumor should lead to a diagnosis of leiomyosarcoma. Until very recently, the existence of leiomyomas in deep soft tissue has been disputed. Using presently available data, this review examines criteria for malignancy in smooth muscle tumors in each of the following anatomic compartments: deep soft tissue, including retroperitoneum/abdominal cavity; skin; subcutis; and external genitalia. Distinct criteria for assessing malignancy should be applied to smooth muscle tumors at each of these locations.