CD56 and P16 expression in the differential diagnosis of papillary thyroid carcinoma and benign follicular neoplasms.

e18057 Background: The differential diagnosis between papillary carcinoma and follicular thyroid neoplasia is sometimes very difficult, for example, in cases of follicular subtypes of papillary carcinoma and follicular carcinoma or adenomas. Rarely, the pathologist studies core needle biopsies or incidentally finds ectopic thyroid tissue in patients with papillary thyroid carcinoma, which may lead us to suspect metastasis. In these situations, immunohistochemistry is an important tool. Methods: A prospective study was conducted, evaluating a total of 51 thyroid neoplasms in 38 women and 13 men, ranging in age from 17 to 79 years, with a mean age of 52.3 years. Immunohistochemical staining for CD56 and p16 was performed on 38 papillary carcinomas, 8 follicular adenomas, 2 oncocytic cell adenomas, 2 adenomas with papillary architecture, and 1 NIFTP. Data were stored in a Microsoft Excel database and analyzed using chi-square tests in SPSS version 29.0. Results: p16 staining positivity and loss of CD56 expression were statistically significant (p<0.01) for the papillary carcinoma group, both separately and in a double panel (p<0.01); likewise, p16 negativity and preservation of CD56 positivity were statistically significant (p<0.01) for benign follicular neoplasms, both separately and in a double panel (p<0.01). A second set of comparisons was performed between follicular subtypes of papillary thyroid carcinoma versus benign follicular neoplasms using a double panel, obtaining statistically significant results (p<0.05). Conclusions: p16 expression becomes a highly sensitive marker for malignancy, while the loss of CD56 expression is highly specific for malignancy. The use of the CD56 and p16 dual panel is statistically significant for differentiating between papillary thyroid carcinomas and their follicular subtypes from benign follicular neoplasms, making it an important support tool for cases that are difficult to differentiate using immunohistochemical staining techniques available in our country. CD56 expression by immunohistochemistry. Follicular Neoplasm Benign Papillary Carcinoma Total Negative 0 20 20 Positive 10 0 10 Total 10 (33.3%) 20 (66.7%) 30 (100%) Chi-square test: p<0.001.