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Pleuropulmonary blastoma (PPB) is a rare and highly aggressive tumor in children and is distinct from ordinary pulmonary blastoma or carcinosarcoma of the lung. This report describes the phenotypical characteristics of seven PPB tumors. The patients were five females and two males, all diagnosed in their third year of life. Five tumors were located within the pulmonary parenchyma, one in the mediastinum, and one involved both lung and mediastinum. Two children are alive and five have died of disease. Histologically, PPB showed a diffuse proliferation of undifferentiated blastemal cells with additional areas of chondroblastic foci (six tumors), storiform pattern (three tumors), alveolar pattern (one tumor), and lipoblastic differentiation (one tumor). Immunohistochemically, tumor cells were positive for vimentin in five of five tumors, histiocytic markers (A1AT and A1ACT, four of six cases; lysozyme, two of six; KP1, three of five) and myogenic markers (desmin, four of six; HHF35, five of six). S-100 was expressed in the chondroblastic areas (in three of four cases). Epithelial membrane antigen and cytokeratin were positive only in epithelial or mesothelial cells entrapped in the tumor. Ultrastructural examination demonstrated a similarity between the proliferating cells in PPB and those seen in malignant fibrous histiocytoma; that is, PPB tumor was mainly composed of a mixture of primitive, fibroblastic, myofibroblastic, histiocytoid, and fibrohistiocytoid cells. Also identified were cells with rhabdomyoblastic differentiation (two tumors) showing thick and thin filaments with Z-discs. In conclusion, PPB showed phenotypical diversity and was composed of MFH-type cells and cells with more specific mesenchymal differentiation.

Pathologic features of eight cases of undifferentiated (embryonal) sarcoma of the liver (USL) in childhood were studied. Light microscopic examination showed a diffuse growth of spindle cells with occasional polygonal cells and multinucleated giant cells and also revealed focal areas of storiform pattern in four tumors, cambium layer formation in one tumor, and alveolar arrangement in one tumor. Immunohistochemical study showed positive staining of proliferating cells for suggestive histiocytic markers (A1AT in 6/6, A1ACT in 5/6, lysozyme in 4/6, and KP1 in 4/6) and for muscle markers (desmin in 4/6 and HHF35 in 3/6). Ultrastructural examination demonstrated that the individual tumors were composed of a mixture of cells having fibroblastic, histiocytoid, fibrohistiocytoid, myofibroblastic, and undifferentiated (primitive mesenchymal) morphologies. Also identified were cells with definite myoblastic morphology in three tumors: leio-myoblastic in one and rhabdomyoblastic in two. In conclusion, the tumor cells in USL show phenotypical diversity comparable to those of malignant fibrous histiocytoma with or without additional rhabdomyosarcomatous or leiomyosarcomatous differentiation.