Differential expression of cytokeratins in follicular variant of papillary carcinoma: An immunohistochemical study and its diagnostic utility

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 CKI9 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.