MSH: A malignant neoplasm of the lung composed chiefly or entirely of immature undifferentiated cells (i.e., blast forms) with little or virtually no stroma. (From Stedman, 25th ed),NCI: A malignant neoplasm of the lung composed of tubular structures and immature mesenchymal elements, which may differentiate towards skeletal and smooth muscle, cartilage or a combination of muscle and cartilage. This is a nodular tumor found in the periphery of the lung. It can occur at any age. The prognosis is related to the stage of the disease at the time of resection. Pulmonary blastoma is divided into two subtypes: epithelial predominant and biphasic.