Cytokeratin subtypes in thyroid tumours: Immunohistochemical study with emphasis on the follicular variant of papillary carcinoma

Objective: The follicular variant of papillary thyroid carcinoma (FV-PTC) may be difficult to differentiate from other thyroid neoplasms with follicular architecture such as follicular adenoma, follicular carcinoma, and dominant nodules in nodular goitre. Obvious differences in treatment and expected outcome between these lesions mandate that the distinction between them be made accurately. To decrease the subjectivity of this differential diagnosis, we undertook this study to determine if any difference in cytokeratin profile exists between FV-PTC and other follicular lesions of the thyroid gland. Design: Immunohistochemical analysis based on a retrospective pathology review. Setting: Vancouver General Hospital, Vancouver, BC. Methods: The files of the Vancouver General Hospital anatomic pathology laboratory were searched for cases of typical papillary thyroid carcinoma (PTC), FV-PTC, follicular adenoma, follicular carcinoma, and nodular goitre. The slides and reports were reviewed, and those cases with confirmed diagnosis and adequate tissue were selected for inclusion in the study. Monoclonal antibodies to cytokeratin 19 (CK19), 20 (CK20), and 7 (CK7) were applied to formalin-fixed, paraffin-embedded tissue sections. Results: All cases of PTC, including FV-PTC, as well as all follicular adenomas, follicular carcinomas, and nodular goitres, stained positively for CK7 and, except for a single follicular carcinoma, were negative for CK20. CK19 decorated almost all PTCs, including FV-PTC, although the staining was sometimes focal. The majority of the cases of follicular adenoma, follicular carcinoma, and nodular goitre were negative or showed focal staining with CK19, although occasional cases showed diffuse positivity. Conclusions: CK19 strongly stains the majority of PTC, including FV-PTC, in a diffuse manner. However, overlap with the. staining seen in other follicular lesions limits its utility in a routine diagnostic setting.