Absent CD56 expression in papillary thyroid carcinoma: A finding of potential diagnostic value in problematic cases of thyroid pathology

Background: There are inter-observer disagreements between papillary thyroid carcinoma (PTC) with other follicular lesions of thyroid in aspect of diagnosis. CD56 is present on follicular epithelial cells of the normal thyroid. We evaluated the diagnostic value of CD56 expression in PTC, follicular thyroid lesions, and follicular thyroid neoplasms. Materials and Methods: Seventythree cases diagnosed as follicular lesions and 73 cases diagnosed as PTC were stained with CD56 marker. A positive membranous immunostaining in more than 10% of the neoplastic cells qualified the case as “positive (+)” for CD56. Results: CD56 expression was seen in 70 samples of non-papillary carcinoma lesion (95.8%) versus one case of PTC (1.3%) (P < 0.001, Chi-square). Therefore, CD56 was 98.6% sensitive and 95.8% specific in distinguishing PTC from other follicular thyroid lesions. Conclusion: CD56 is both a sensitive and specific marker for differentiating PTC from other follicular lesions of thyroid singly but it may be better to use a combination of markers for clinical evaluation of patients.