الفهرس 
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Abstract
Background and aim of the work: Diagnostic difficulties in thyroid lesions are caused by morphological similarities of papillary hyperplasia versus encapsulated papillary carcinoma and follicular adenoma versus minimally invasive follicular carcinoma. The purpose of this study was to evaluate the diagnostic value of CD56, galectin-3, S100 and Rb-1 immunohistochemical staining in distinguishing benign thyroid lesions from malignant lesions. Materials and Methods: Immunohistochemistry using the chain polymer conjugated method was performed for antibody against CD56, galectin-3, S100 and Rb-1 in 129 cases of both benign and malignant thyroid lesions. The data were submitted for statistical analysis. Results: Statistical analysis showed significantly different expressions of CD56 in PTC versus other lesions of the thyroid gland except anaplastic carcinoma (AC). The sensitivity and specificity of CD56 for diagnosing PTC vs other thyroid lesions were 100% and 92% respectively. Galectin 3 expression was much higher in malignant lesions than benign lesions with prevalence in papillary carcinoma. Rb-1 can significantly differentiate between the follicular carcinoma and follicular adenoma with the highest sensitivity. However, it doesn’t show any significance in differentiation between PTC including FVPTC and follicular carcinoma. As regard immunohistochemistry for monoclonal S100, all cases, whether benign or malignant were negative including the papillary carcinoma. Conclusion: CD56 is a negative diagnostic marker for papillary thyroid carcinoma with the highest sensitivity and specificity followed by galectin-3. Regarding the differentiation between benign and malignant thyroid lesions, Rb-1 has the highest sensitivity, while CD56 and galectin-3 have the highest specificity, so a panel using Rb-1with either CD56 or galectin-3 is recommended.