Papillary carcinoma of the thyroid. A histochemical, immunohistochemical and ultrastructural study with special reference to the follicular variant

Abstract

Twenty-one papillary thyroid carcinomas (PTCs), grouped into predominantly papillary (14 cases), predominantly follicular (5 cases), and extremely follicular, i.e., follicular variant (2 cases) types, were studied in comparison with three cases each of follicular lesions including follicular carcinoma, follicular adenoma, adenomatous goiter and Graves' disease. Histochemical, immunoperoxidase, and electron microscopic analyses demonstrated no remarkable differences between the predominantly papillary and predominantly follicular PTCs, but the presence of common characteristics distinct from those of the follicular lesions. These two types of PTCs showed less glycogen, more mucoid material, more epidermal keratin, less thyroid hormone with relative predominance of T3 over T4, and more interdigitating reticulum cells (IDCs) than most of the follicular lesions. Ultrastructurally, the tumor cells of these PTCs had markedly irregular nuclei, a vesicular chromatin pattern, and small basally located lysosomes, in contrast with the cells in the follicular lesions which had smooth round nuclei, more heterochromatin, and apical or dispersed lysosomes of various sizes. The follicular variant PTCs showed some mixed features, such as glycogen in the follicular portion and mucoid material in metastatic papillary foci, positive keratin and IDCs but greater amounts of thyroid hormone, and a rather intermediate type of ultrastructure with only mildly irregular but vesicular nuclei and large apical as well as small basal lysosomes. These findings cytologically support the WHO definition of papillary carcinoma that includes tumors with variable mixtures of papillary and follicular patterns. However, separate consideration may be necessary with regard to the follicular variant.