Soft tissue sarcomas of childhood: the differential diagnostic dilemma of the small blue cell.

In its histologic features, embryonal rhabdomyosarcoma (RMS), the prototype of malignant soft tissue tumors in childhood, summarizes the problems associated with the diagnosis of this entire group of neoplasms. Many of the tumors that do not fulfill the criteria for RMS have been designated "sarcomas of uncertain histogenesis." The introduction of the concept of a soft tissue equivalent of Ewing's sarcoma may have eased the semantic anxiety without improving our conceptual understanding. It is thought that the embryonal RMS, Ewing's sarcoma, and other are derived from a primitive mesenchymal cell. Another separate category of "small blue cell tumors" are those which presumably originate from the primitive neuroepithelium. Some of the diagnostic terms applied to this category are "neuroepithelioma," "medulloepithelioma," and "peripheral neuroblastoma." Because most of these tumors are hormonally inactive and electron microscopy is not performed, the diagnosis is infrequently considered or proved. The recently described small cell tumor of thoracopulmonary origin is likely a malignant neuroepithelial neoplasm. Hematopoietic tumors, such as non-Hodgkin's malignant lymphomas, granulocytic sarcoma, and malignant histiocytosis, may appear in the soft tissues as the initial manifestation of these system diseases. A final group of malignant soft tissue tumors are the fibrohistiocytic ones with a biphasic pattern of small round cells and spindle cells. It now has become increasingly difficult for the pathologist to satisfy his clinical colleagues with the diagnosis of "undifferentiated malignant tumor" in a child.