Lipoleiomyosarcoma (well-differentiated liposarcoma with leiomyosarcomatous differentiation): a clinicopathologic study of nine cases including one with dedifferentiation.

Leiomyosarcomatous (LMS) differentiation is a rare event in liposarcoma (LPS) and may consist of either well-differentiated liposarcoma (WDL) with an intrinsic smooth muscle component, so-called "lipoleiomyosarcoma," (L-LMS) or dedifferentiated liposarcoma having smooth muscle differentiation in the dedifferentiated zones. The latter are high-grade sarcomas, whereas the behavior of the former group is uncertain. Specifically, it is not clear whether the presence of LMS negatively affects the prognosis. We present our experience with nine cases, the largest to date. The patients (seven male, two female) ranged in age from 42 to 65 years (mean 54 years). The tumors were usually large (2 to >40 cm [mean 17 cm]) tumors in the retroperitoneum (two cases), paratesticular-inguinal region (three cases), mediastinum (one case), lung (one case), abdomen (one case), and popliteal fossa (one case). The nine cases qualified as L-LMS and showed typical WDL with a multifocal, gradual transition into smooth muscle areas. The latter areas accounted for a variable portion of the lesions (range 5-90%) and were of low cellularity, mild to moderate nuclear atypia, and low mitotic activity. These areas seemed to arise from or blend with the smooth muscle in the walls of large vessels within the tumor. One case showed areas of dedifferentiation consisting of actin and desmin-negative, high-grade sarcoma. Follow-up in seven cases (range 26-312 months; mean 119 months) showed multiple local recurrences in seven patients and no metastases. Three patients are currently without evidence of disease (follow-up duration 26-312 months; mean 144 months) and four patients are alive with progressive disease (follow-up duration 60-132 months; mean 99 months). Our study suggests that L-LMS is a dual lineage sarcoma as evidenced by the fact that the smooth muscle component is often multifocal, not necessarily found in close association with the atypical changes in fat, and seemingly originates from atypical ("in situ") changes in the vessel wall. The LMS component, which is typically low grade, does not adversely affect the overall behavior of the tumor, which is similar to that of conventional WDL. LMS in L-LMS should not be misconstrued as evidence of low-grade dedifferentiation, a phenomenon that identifies a more unstable and potentially metastasizing lesion.