Objective: Thyroid nodules which are common in population consist of mostly benign nodules and less malignant neoplasms. Differentiating malignant nodules from each other arises a drastic problem in making a correct diagnosis Thus,this study was done to investigate the role of CD56 and its specificity and sensitivity in different types of  thyroid neoplasms especially papillary carcinoma.

Methods: 73 paraffin embedded-blocks of thyroid massses (nodules) were studied for CD56 immunohistologically from Imam Khomeini hospital ,Sari,Mazandaran.The 4-scaled semiquantitative method (0-3) was applied to estimate CD56 expression in tumor cells less than 10% (0) or negative staining, 10-33% (1), 33-66% (2), more than 66% (3).

Results: In this study 24.7% were male and 75.3% were female. 13.7% ,68.5 % and 17.8% of patients were less than 20, between 20- 50,and over 50 years old respectively.Simultaneously 50(68.5%) patients had both right and left lobes nodules.12(16.4%) ,6(8.2%)and 1(1.4%)  patients had nodules in right and left lobes and isthmus respectively.Consequently, 4 (5.5%)patients showed left and right lobes ,and ithsmus nodules simultaneousely. There was no statistically significant difference between the sexes and ages and also anatomical regions in patients with and without PTC. 30 out of  the 73 patients had papillary carcinoma(PTC) which 4 out of them expressed CD56 while 37 out of 43(86%) patients who had non-PTC showed positive results for CD56.Sensivity and specificity of negative result of CD56,positive predictive value(PPV) and negative predictive value(NPV) in the diagnosis of papillary carcinoma were 86.6%,86%,81.2% and 86.3% respectively.   

Conclusion: Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and overall accuracy (OA) of  negative result of CD56  in the diagnosis of PTC were  86.6%, 86.04%, 81.25%, 90.24% and 86.3% respectively. This study showed that CD56 was a valuable sensitive and specific marker in differentiating PTC from other thyroid tumors.