A rare pediatric intrathoracic neoplasm. The tumor arises from the lung, pleura, or both, and appears to be purely mesenchymal in phenotype. It lacks malignant epithelial elements, a feature that distinguishes it from the classic adult-type pulmonary blastoma. It arises during fetal lung development and is often part of an inherited cancer syndrome. The tumor contain both epithelial and mesenchymal cells. Early in tumorigenesis, cysts form in lung airspaces, and these cysts are lined with benign-appearing epithelium. Mesenchymal cells susceptible to malignant transformation reside within the cyst walls and form a dense layer beneath the epithelial lining. In a subset of patients, overgrowth of the mesenchymal cells produces a sarcoma, a transition that is associated with a poorer prognosis. Some patients have multilocular cystic nephroma, a benign kidney tumor.
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