Preoperative Risk Stratification of Follicular-patterned Thyroid Lesions on Core Needle Biopsy by Histologic Subtyping and RAS Variant-specific Immunohistochemistry.

Follicular-patterned lesions often have indeterminate results (diagnostic category III or IV) by core needle biopsy (CNB) and fine needle aspiration (FNA). However, CNB diagnoses follicular neoplasm (category IV) more frequently than FNA. Therefore, we aimed to develop a risk stratification system for CNB samples with category III/IV using immunohistochemistry (IHC). The specificity of the RAS Q61R antibody was validated on 58 thyroid nodules with six different types of RAS genetic variants and 40 cases of RAS wild-type. We then applied IHC analysis of RAS Q61R to 207 CNB samples with category III/IV in which all patients underwent surgical resection. RAS Q61R IHC had 98% sensitivity and 98% specificity for detecting the RAS p.Q16R variant. In an independent dataset, the positive rate of RAS Q61R was significantly higher in NIFTP (48%) and malignancies (45%) than in benign tumors (19%). The risk of NIFTP/malignancy was highest in the group with nuclear atypia and RAS Q61R expression (86%) and lowest in the group without both parameters (32%). The high-risk group with either nuclear atypia or RAS Q61R had 67.3% sensitivity, 73.4% specificity, 75.2% positive predictive value, and 65.1% negative predictive value for identifying NIFTP/malignancy. We conclude that RAS Q61R IHC can be a rule-in diagnostic test for NIFTP/malignancy in CNB category III/IV results. Combining of the histologic parameter (nuclear atypia) with RAS Q61R IHC results can further stratify CNB category III/IV into a high-risk group, which is sufficient for a surgical referral, and a low-risk group sufficient for observation.