Leiomyosarcoma of somatic soft tissues in childhood: an immunohistochemical analysis of six cases with ultrastructural correlation.

Leiomyosarcoma (LMS) of soft tissue is a rare tumor in children. Although LMS may exhibit clinical and histologic features that are typical of smooth muscle neoplasms in adults, they may often be confused with or resemble tumors of presumed fibroblastic, myofibroblastic, or rhabdomyoblastic differentiation. As a result, the diagnosis of LMS in children is often difficult to establish with confidence. To address this problem, we analyzed the immunohistochemical features of six LMS in children (one of deep soft tissue of an extremity, one of paravertebral tissue, one of the retroperitoneum, two of oropharyngeal soft tissue, and one of subcutis); the ultrastructural features of four of these tumors were also studied. Histologically, each of the three tumors of deep soft tissue and one of the retromolar trigone were composed of pleomorphic spindle cells arranged in interweaving fascicles. In contrast, the subcutaneous tumor and the lesion of the hard palate had an epithelioid appearance. Ultrastructural features were typical of adult type LMS. Immunohistochemically, these neoplasms were diffusely reactive for vimentin, while each was negative for cytokeratin, epithelial membrane antigen, and S-100 protein. Desmin was present in all cases, but was expressed only focally in three; a similar pattern of staining was noted for muscle-specific actin, although staining was generally noted in a larger population of cells. alpha-1-Antichymotrypsin was found in five tumors, and cathepsin B reactivity was encountered in four cases. Leu-7 antigen and myelin basic protein were coexpressed by one tumor, but neither was found in the remaining cases. These results indicate that immunohistochemical detection of desmin and muscle-specific actin may be useful in the differential diagnosis of spindle or epithelioid cell proliferations in childhood when ultrastructural analysis in unavailable.