Diagnostic utility of cytokeratin 19 and CD56 in the differentiation of thyroid papillary carcinoma from its mimics

Objectives: Thyroid carcinomas are the most common endocrine malignancy, and more than 95% of cases originate from the thyroid follicular epithelial cells. Over the past few decades, many variants of papillary thyroid carcinoma (PTC) have been described. Among them, the most common variant is the follicular variant of PTC (FVPTC). The key diagnostic criteria of PTC are the presence of its characteristic nuclear features. However, the focal presence of these features in other thyroid lesions can cause a diagnostic dilemma. For example, encapsulated FVPTC has a thick capsule and distinct follicular pattern making it difficult to distinguish from a follicular adenoma. The diagnosis becomes challenging even for experienced pathologists worldwide due to such overlapping morphological features. The aim of this study was to examine the clinical spectrum of various thyroid lesions and to evaluate the utility of CD56 and cytokeratin 19 (CK 19) by immunohistochemical analysis as diagnostic markers in distinguishing PTC from other histologic mimics. Material and Methods: Retrospective observational study done on the paraffin blocks of 102 cases of various thyroid lesions received in the pathology department at a single tertiary care center. Clinical data like age, gross findings like tumor size, and type of surgery were collected. The histopathological data were also collected from the archives of the department of pathology. Immunohistochemistry (IHC) workup for CD56 and CK 19 expression was evaluated and graded based on the intensity and percentage of positively staining cells. Results: One hundred and two cases of benign and malignant thyroid lesions, which were presented to the department, were analyzed. The mean age of the participants was 43.1 years. Many of the participants were female (80.4%). Most cases were PTC (20.6%) and FVPTC (20.6%). The next most common diagnosis was follicular adenoma (12.7%). CD56 was predominantly negative in PTC (92.3%) when compared to its mimics that were positive for CD 56 (7.7%). CK 19 was predominantly positive in PTC (96.2%) when compared to its mimics that were negative for CK 19 (3.8%). Conclusion: The basic IHC panel for an accurate diagnosis of PTC should include CK 19 as a positive marker and CD56 as a negative marker for equivocal histological diagnosis. This differentiation of PTC and its variants from its mimickers is essential for prognosis and instituting optimal treatment.