MALIGNANT SMOOTH-MUSCLE TUMORS PRESENTING AS MEDIASTINAL SOFT-TISSUE MASSES - A CLINICOPATHOLOGICAL STUDY OF 10 CASES

Background. Smooth muscle tumors presenting as mediastinal soft tissue masses are extremely rare and often are mistaken for other neoplastic conditions. Methods. Ten cases of patients with malignant smooth musc le tumors presenting as mediastinal soft tissue masses were studied an d correlated with their clinical behavior. Tissues were examined histo logically and with immunohistochemical stains in all cases, and by ele ctron microscopy in two cases. Results. The patients' ages ranged from 26 to 71 years (mean, 56 years); three were women, and seven were men . Three cases were located in the anterior mediastinum and seven in th e posterior mediastinum. The patients with anterosuperior mediastinal tumors all presented with signs and symptoms referable to their lesion s; the patients with posterior mediastinal masses (with the exception of one) were all asymptomatic. Grossly, the lesions were well circumsc ribed and unencapsulated, ranging from 6 to 18 cm in greatest dimensio n and showed a homogeneous, rubbery cut surface with prominent cystic and myxoid areas. The tumors in all patients appeared to arise from th e soft tissues within the mediastinum and were unrelated to adjacent s tructures. In three patients, the tumors compressed and displaced the esophagus without infiltrating its wall, and in one patient, the tumor was found in close proximity, although unattached, to a large vessel. Histologically, the lesions exhibited a spectrum of morphologic appea rances that ranged from low grade leiomyosarcoma with mild-to-moderate nuclear atypia and low mitotic activity (< 3/10 high power fields [HP Fs]), to high grade tumors with marked nuclear pleomorphism, extensive areas of necrosis, and high mitotic activity (> 10 mitoses/10 HPFs), One case was characterized by a striking epithelioid morphology with l arge, round cells arranged in small clusters; another was associated w ith an incidental microscopic focus of thymic seminoma in the adjacent thymus. Immunohistochemical stains in all cases showed positive label ing of the tumor cells with smooth muscle actin, desmin, and vimentin antibodies. Electron microscopy in two cases showed features of smooth muscle differentiation, i.e., spindle cells surrounded by basal lamin a material, immature cell junctions, and abundant intracytoplasmic fil aments with focal condensations. All patients were treated with surgic al excision. On follow-up, three patients with Stage IIIb and IVa tumo rs died 2-7 years after surgery, and two patients with Stage Ib and II b were alive and well 4 and 6 years after surgery, respectively. Concl usion. Leiomyosarcomas may arise as primary tumors originating from me diastinal soft tissues in both anterior and posterior compartments. Be cause of their large size and frequent areas of cystic and myxoid dege neration, they may be confused histologically with neural or other neo plasms. As with their counterparts in other soft tissue locations, his tologic grade and clinical stage are the most useful parameters for as sessing prognosis.