Infantile rhabdomyofibrosarcoma: a high-grade sarcoma distinguishable from infantile fibrosarcoma and rhabdomyosarcoma.

Three patients with a predominantly fibrosarcomatous, desmoplastic, and fascicular spindle-cell sarcoma, resembling that of infantile fibrosarcoma but with ultrastructural and immunocytochemical evidence of rhabdomyoblastic differentiation, are described. The tumors were compared light and electron microscopically and immunocytochemically with six classical cases of infantile fibrosarcoma of the desmoplastic type. The three tumors occurred in two girls and one boy who were between 1 and 3 years of age. Ultrastructurally, the tumor cells primarily demonstrated fibroblastic and myofibroblastic features, but there were also tumor cells with sarcomere-like structures. The spindle-shaped, fibroblast-like tumor cells expressed vimentin, desmin, and smooth muscle-specific and sarcomere-specific actins. In one tumor peculiar intracytoplasmic inclusions were observed by light and electron microscopy. The immunocytochemical analysis indicated that these bodies contained all types of thin and intermediate filaments. In short-term cell cultures the tumor cells of two examined tumors revealed immunoreactivity for vimentin, smooth muscle-specific actin, and desmin. Cytogenetically, both tumors displayed a similar chromosomal pattern. One tumor contained two abnormal clones with monosomy 19 and monosomy 22, respectively, and the other tumor revealed a single clone with monosomy 19. In addition, both tumors contained clones with a normal karyotype. Two of the patients with metastatic spread died within 2 years of the primary operation. The third patient is alive with a local recurrence. The three tumors described here deviate from infantile fibrosarcoma in terms of their clinical, ultrastructural, immunocytochemical, and cytogenetic characteristics. The descriptive term "infantile rhabdomyofibrosarcoma" is suggested. The present investigation calls for detailed studies of tumors diagnosed as infantile fibrosarcomas that have metastasized and followed a fatal course.