Diagnostic Utility of Cytokeratin 19 in Differentiating Papillary Thyroid Carcinoma from Follicular Patterned Thyroid Lesions -An Immunohistochemical Study

Background: Papillary thyroid carcinoma (PTC) is the most common malignant tumour of the thyroid gland, accounting for approximately 80–85% of all thyroid malignancies. Its diagnosis relies on distinctive nuclear features; however, these may be subtle or equivocal in follicular-patterned lesions, posing diagnostic challenges. Cytokeratin 19 (CK19) immunohistochemistry has been proposed as a useful adjunct in diagnostically challenging cases. The aim is to evaluate the diagnostic utility of CK19 immunohistochemistry in differentiating Papillary thyroid carcinoma and its follicular variant from other follicular-patterned thyroid lesions. Material and Methods: This retrospective observational study was conducted in the Department of Pathology, Dr. D. Y. Patil School of Medicine, Nerul, Navi Mumbai, over two years (January 2024 – December 2025). A total of 28 thyroidectomy specimens comprising PTC (n=4), FVPTC (n=4), follicular adenoma (FA, n=14), multinodular goitre (MNG, n=2), and non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP, n=4) were included. CK19 expression was graded semi- quantitatively from 0 to 4+. Results: All eight cases of PTC/FVPTC showed Strong, diffuse, membranous and cytoplasmic positivity (4+) CK19 positivity. Benign lesions demonstrated absent or focal staining. NIFTP showed heterogeneous expression without diffuse strong positivity. CK19 demonstrated 100% sensitivity, specificity, positive predictive value, and negative predictive value in this cohort (Fisher's exact test, p<0.0001). Conclusion: CK19 immunohistochemistry is a reliable adjunct tool for differentiating Papillary thyroid carcinoma and its variants from benign follicular-patterned thyroid lesions, particularly in cases with equivocal morphological features.

Keywords: Papillary thyroid carcinoma; Cytokeratin 19; Follicular-patterned thyroid lesions; Immunohistochemistry; NIFTP.