Retinoblastoma expression in thyroid neoplasms

Retinoblastoma (Rb) mutation in thyroid neoplasia has been identified in a few molecular studies; however, the utility of Rb immunohistochemistry in d istinguishing benign and malignant thyroid lesions has not been documented in formalin-fixed, paraffin-embedded tissues. The present study investigate d Rb immunohistochemistry in a series of 111 formalin-fixed, paraffin-embed ded benign and malignant thyroid lesions. All of the major histologic subty pes were included to detect any heterogeneity in Rb-l expression that might influence the diagnostic utility of this technique or further elucidate th e pathogenesis of thyroid neoplasia among the categories. Altogether, 34 fo llicular adenomas, 9 follicular carcinomas, 7 Hurthle cell adenomas, 5 Hurt hle cell carcinomas, 23 papillary carcinomas (8 of which were follicular va riants), 4 insular carcinomas, 4 anaplastic carcinomas, 6 medullary carcino mas, and 19 nodular goiters were analyzed. Avidin-biotin immunohistochemist ry was performed using the Dako Rb-l clone. Pronase digestion was introduce d into the epitope retrieval protocol to eliminate false-positive cytoplasm ic staining. Nuclear immunoreactivity was assessed as positive if 10% or mo re of thyroid epithelial nuclei stained positively, and conversely as negat ive. The majority of benign non-Hurthle thyroid lesions, whether hyperplast ic or neoplastic, retained Rb nuclear immunopositivity in most cells (51 of 53 cases [96%]). Conversely, malignant thyroid neoplasms lacked Rib immuno reactivity in the majority (42 of 51 cases [82%]), including all papillary carcinomas (23 of 23) and almost all follicular carcinomas (8 of 9 [89%]). Virtually all Hurthle cell neoplasms were negative (11 of Ii [92%]), whethe r benign or malignant. In conclusion, Rb imnunohistochemistry can aid in th e distinction between benign and malignant thyroid lesions in conjunction w ith morphology. This seems to be most applicable to the often problematic d ifferentiation between follicular adenoma and the follicular Variant of pap illary carcinoma (P <.0001; sensitivity and specificity, 100%) or minimally invasive follicular carcinoma (P = .0007; sensitivity, 89%; specificity, 1 00%).