Malignant tumors with an intimate mixture of cellular components from two germ layers, such as epithelium and mesothelium, in varying proportions, are of special interest because of their rarity, and because their tissues contain surprising and bizarre combinations of cell structures. Such tumors originating in the thyroid gland, in the mammary gland, and elsewhere, have been reported as carcinosarcomas (Mason and Wells; Küchens; Saltykow). Wegelin's recent review of these tumors of the thyroid mentioned only a few, and the structure of these varied considerably. Antedating Wegelin's review by several years was the report by Schuppisser in which he stated, as have others, that true carcinosarcomas of the thyroid are rare. He mentioned the two tumors reported by Förster, those reported by Kocher, Jr., Simmonds, Albrecht, and Schmorl, and one in a dog reported by Wells. These summarizing statements and those in the recent account by Mason and Wells indicate how few indeed are the reports of malignant mixed cell tumors of the thyroid.
The variations in histologic structure seem to depend upon the degree of differentiation of the epithelial and mesothelial tissues and the proportion of each entering into the composition of the primary growth and its metastases. Some of the accounts report a fairly complete separation of the tissue derivatives in the primary growth, in the recurrence, or in the metastases, so that in these various places elements of only one of the two germ layers were found.