Cytokeratin 19 is the lowest molecular weight cytokeratin and is found in a diverse range of normal epithelia and tumors. 1 On histologic sections, cytokeratin 19 has been expressed in all tumor cells of thyroid papillary carcinomas, but it has been absent or only focally present in follicular neoplasms and adenomatous goiters. 2-5 However, on cytologic smears there have been no reports concerning the expression of cytokeratin 19. Therefore, we investigated whether immunocytochemical staining using the anticytokeratin 19 antibody in cytologic smears is helpful in distinguishing among papillary carcinoma, adenomatous goiter, and follicular neoplasms of the thyroid.
From the files of the Cytology Laboratory, Kawasaki Medical School Hospital, we collected alcohol-fixed, Papanicolaou-stained smears imprinted from surgically resected thyroid lesions collected between January 1992-December 1997, which included 22 papillary carcinomas, 24 adenomatous goiters, 8 follicular adenomas, and 2 follicular carcinomas. The diagnoses of all cases were confirmed by histologic examination. Two cases of follicular carcinoma were of widely invasive type and revealed metastasis to the bone. After the smears were destained by ethyl alcohol, they were immunocytochemically stained using anti-cytokeratin 19 antibody (1:200, Progen, Heidelberg, Germany) and ENVISION reagent (Dako Japan, Kyoto, Japan). Negative controls were obtained by replacing the primary antibody with Tris-buffered saline. We used paraffin sections of papillary carcinoma as positive controls. The degree of immunoreaction was rated on a five-point scale: -, no staining; ฯฉ, 10% or less of cells have positive staining; 2ฯฉ, between 10-50% of cells have positive staining; 3ฯฉ, between 50-90% of cells have positive staining; and 4ฯฉ, 90% or more have positive staining.
Most cases of papillary carcinoma were strongly immunopositive for cytokeratin 19 (Table I). The staining pattern was predominantly pancytoplasmic. Central areas of the large clusters tended to be weakly positive or negative. The immunoreactivity for cytokeratin 19 was stronger in large tumor cells with abundant cytoplasm than in small tumor cells. Most follicular epithelial cells in adenomatous goiter