Diagnostic utility of CD56 immunohistochemistry in papillary carcinoma of the thyroid.

Diagnosis of papillary thyroid carcinoma (PTC), in many but not all cases, is an easily achievable diagnosis with almost minimal interobservable variability between pathologists. However, some cases of PTC, particularly the follicular variant, are quite challenging and show wide interobservable variability even among expert thyroid pathologists. Since proper diagnosis of PTC is crucial as it affects patients' clinical management and prognosis, indications of PTC must be clearly apparent to be an objective rather than a subjective diagnosis. Unfortunately, to date, immunohistochemistry and molecular studies have failed to fully solve this problem. In this study, we assessed the protein expression and loss using antibodies against CD56 in normal follicular thyroid epithelium, follicular thyroid lesions, and follicular thyroid neoplasms in an attempt to evaluate its diagnostic value. A total of 185 cases were studied with tissues from 75 carcinomas (72 papillary, 2 follicular, 1 Hürthle cell) and 35 adenomas (32 follicular and 3 Hürthle cell) evaluated by immunohistochemistry for the expression of this marker. Non-neoplastic thyroids included 65 cases: nodular hyperplasia (n=25), thyrotoxic hyperplasia (Grave's disease) (n=5), lymphocytic thyroiditis (n=19), and Hashimoto's thyroiditis (n=6). Ten cases of normal thyroids from radical laryngectomies for laryngeal squamous cell carcinomas were also studied. The marker pattern and intensity of staining were scored. Positive expression of the markers in 10% or more of follicular epithelium within the tumor or lesional cells was considered positive. An expression of <10% was considered to be negative. Diffuse CD56 expression was consistently present in normal, lesional, and neoplastic follicular epithelium, except for PTC, including the follicular variant. We concluded that CD56 is of value to distinguish PTC from other thyroid follicular pathology/histology with a sensitivity of 100% and a specificity of 100%. We suggest that CD56 is extremely useful in the diagnosis of PTC, including the follicular variant, and to distinguish it from other follicular cell-derived thyroid tumors/lesions. Application of CD56 by a group of expert pathologists on a larger series of follicular thyroid neoplasms of uncertain malignant potentials may potentially provide an objective diagnostic tool.