The follicular variant of
papillary carcinoma (FVPTC) is characterized by follicular growth pattern and
tumor cells with appropriate nuclear features of
papillary carcinoma. However, occasionally these lesions may show focal or multifocal instead of diffuse distribution of nuclear features of
papillary carcinoma. Such lesions can be underdiagnosed as benign follicular nodule. Previous studies have shown that cytokeratins, especially 19, are helpful in differentiating
papillary carcinoma from other benign and malignant follicular patterned lesions. In this study, we applied
monoclonal antibodies to CK5/6/18, CK18, CK10/13, CK20, CK17, and CK19 to
paraffin sections of
formaldehyde-fixed tissue from 26 cases of FVPTC with multifocal distribution of papillary
cancer nuclei, 10 cases of usual variant of
papillary carcinoma, 1 case of Warthin's
tumor-like
papillary carcinoma, and 2 cases of the columnar cell
carcinoma. CK19 stained strongly and diffusely all cases of
papillary carcinoma. FVPTC cases showed strong staining of the areas with papillary
cancer nuclei in all cases and moderate to strong staining in areas of
tumor without obvious nuclear features of papillary
cancer. Normal thyroid parenchyma adjacent to the
tumor nodule showed focal staining in most cases; however, tissue away from the
tumor nodule failed to show any staining. All cases of usual type of
papillary carcinoma, 2 of columnar cell
carcinoma, and 1 Warthin's
tumor-like
papillary carcinoma showed strong and diffuse staining with CK19 and failed to show any staining of adjacent normal thyroid parenchyma. Similar but less intense staining patterns were seen with CK17 and CK20. The control group, consisting of cases of
follicular adenoma, follicular
carcinoma, and hyperplastic nodule, showed no staining with CK19. We suggest that if one is using immunohistochemistry to aid in the diagnosis of cases of FVPTC with multifocal distribution of nuclear features of papillary
cancer, an antibody panel comprising
CKs 17, 19, and 20 may prove helpful. In addition, we hypothesize that the staining of adjacent nontumorous thyroid parenchyma with CK19, seen only in cases of FVPTC, suggests that some factors secreted/produced by this particular
tumor may lead to modification in
keratin expression of surrounding follicular epithelium.