MATERIALS AND METHODS: We retrospectively studied CD56 expression in 54 benign and 54 malignant thyroid lesions using archival formalin fixed paraffin-embedded tissue blocks for the study period from January 2010 to December 2015, diagnosed in a tertiary hospital.
RESULTS: CD56 was expressed in 52/54 (96.3%) of benign specimens and only 24/54 (44.4%) of malignant ones. The malignant specimens comprised 31 (57.4%) papillary thyroid carcinomas (PTC), 11 (20.3%) follicular carcinomas (FC), seven (13%) medullary thyroid carcinomas (MC), one (1.9%) poorly differentiated carcinoma (PC) and four (7.4%) anaplastic carcinomas (AC). CD56 was not expressed in 28/31 (90.3%) of the PTCs, 1/11 (9.1%) FCs, 1/4 (25%) of ACs while all MCs and the PD were positive. The benign group comprised nodular hyperplasias (29/54), lymphocytic thyroiditis (10/54), follicular adenomas (FA) (14/54) and one hyalinising trabecular tumour. CD56 was expressed in all the benign cases except one FA and one nodular hyperplasia. Thirteen of the 14 FAs were CD56 positive. The difference in expression between benign and malignant tumours was statistically significant as the p value was <0.01.
CONCLUSION: CD56 is a potentially good immunohistochemical marker for differentiating papillary thyroid carcinoma from other benign follicular lesions of the thyroid especially in differentiating follicular variant PTC from FA in equivocal cases.