Synovial sarcoma. A clinical, pathological, and ultrastructural study of 26 cases supporting the recognition of a monophasic variant

Twenty-six cases of synovial sarcoma (14 biphasic, 12 monophasic) were subjected to a clinicopathological study that included electron-microscopic examination of six tumors. Monophasic tumors were composed predominantly of uniform, densely packed, small spindle cells with scant cytoplasm identical to those of the 'stromal' elements of typical biphasic tumors. The arrangement of these cells into narrow interlacing fascicles, forming tight whorls and showing little collagenization, was distinctive for this tumor. Major clinical differences between the two types of synovial sarcoma were the tendency for monophasic tumors to arise in distal extremity locations (seven of 10), and the poorer prognosis of monophasic tumors, 30% surviving 5 years compared to 58% for biphasic tumors. At the ultrastructural level, monophasic tumors and the spindle-cell components of biphasic tumors were identical. Both were composed of spindle or polygonal cells attached by numerous desmosomes. Prominent Golgi, abundant RER, perinuclear microfilaments, and glycogen aggregates were characteristic. Intercellular spaces containing elongated cytoplasmic filopodia were observed consistently, as were fragments of basement membrane-like material. The EM findings concur with those described previously in normal and pathologic synovium, and support a synovioblastic origin for the monophasic variant of synovial sarcoma.