THE PATHOLOGICAL ANATOMY 293 



Carcinoma Simplex. The simplex or fetal type arises from pre- 

 existing adenomas, and it is the most frequent type of thyroid carcinoma, 

 making up probably more than 50 per cent. These are likewise the most 

 malignant and most rapidly growing carcinomas. They may occur at any 

 age. The thyroid cancers seen in childhood and early adult life are usually 

 of this type. They are encapsulated because of their origin from ade- 

 nomas and may show no invasion of the capsule when lung metastases 

 are present. 



Histologically these cancers are characterized by the general absence 

 of colloid and the tendency of the alveoli to form solid closely packed 

 columns of cells. The cell type of this form of carcinoma is usually quite 

 uniform. There may be some differentiation with the formation of rudi- 

 mentary alveoli. 



Adeno-Carcinoma. Adeno-carcinomas include the so-called me- 

 tasizing colloid goiters, arid those tumors in which the alveoli become 

 differentiated into colloid containing spaces. There may be infoldings and 

 plications of the lining epithelium. The epithelium may be cuboidal or 

 columnar. The cells are usually irregular with a tendency to heaping up 

 and desquamation. These tumors may arise from the more differentiated 

 adenomas. They form infiltrating growths. This type of carcinoma is 

 relatively common in old dogs with long standing goiter, although ade- 

 nomas are rarely observed. Cohnheim, Ewald, O., (b) Kocher, T. (e). 



Scirrhus Carcinoma. Scirrhus carcinoma is rare. Several ex- 

 amples have been described where the outlines of thyroid lobes slightly, 

 or moderately enlarged, are preserved. Such glands are extremely 

 hard. On section, the tumor resembles a diffuse, extreme fibrosis, which 

 extends into the non-tumor thyroid without any line of demarcation. 

 Microscopically, in the older parts there may be no glandular elements 

 left. Nearer the periphery there are compressed, irregular strands of 

 epithelial cells, widely scattered in the dense stroma and still nearer the 

 growing border, normal appearing thyroid alveoli may be seen in the 

 developing fibrosis. Marine and Johnson, Patel, v. Recklmghausen. 



Papillary Cystic Carcinoma. Tumors with papillary projections, 

 some low and others high and arborescent, may be distinguished as a 

 group. As a rule they grow slowly. They are not all malignant and the 

 benign form has been designated papillary adenoma. Papillary adenomas 

 may be present at birth. The epithelial types vary from cuboidal to high 

 columnar. The cystic spaces may contain colloid material. Low, Zullig. 



Squamous Cell Carcinoma. Squamous cell carcinomas occasion- 

 ally arise in the thyroid, and as regards structure and general appear- 

 ance resemble squamous cell tumors arising elsewhere. They arise from 

 epiblastic structures included in the thyroid, from the thyroglossal tract 

 or from the branchial clefts. Eberth (a), Vevebely. 



