MON-498 Thyroseq V3 GC for Bethesda III and IV: An Institutional Experience

Background: Molecular testing of thyroid nodules classified as indeterminate on fine-needle aspiration (FNA) is used for patient management. ThyroSeq® v3 genomic classifier is a commercially available test that examines a wide spectrum of genomic alterations in a thyroid FNA sample based on algorithmic analysis and reports test results as either negative (including currently negative) or positive. This study reviews our institutional experience with Thyroseq® V3 to distinguish between benign disease versus cancer in thyroid nodules diagnosed as Bethesda III or IV on cytology. Methods: Thyroid nodules with Bethesda III or IV cytology diagnoses and available Thyroseq® V3 results from 12/17 to 8/19 were retrieved from the pathology database. Cytopathology diagnoses were correlated with molecular testing and histopathology. Results: 416 cases (Bethesda III n=252, Bethesda IV n=164) were retrieved: 295 (71%) were reported as Thyroseq® V3 negative and 121 (29%) as positive. The 82.1% (207/252) benign call rate (BCR) of Thyroseq® v3 for Bethesda III was significantly higher (p<0.001) than that for Bethesda IV, BCR 54% (88 /164). Histopathologic follow-up was available for 128 cases (96 ThyroSeq® v3 positive, 32 ThyroSeq® v3 negative): 57 benign and 71 malignant (including NIFTP). For Bethesda III and IV diagnoses respectively, the test demonstrated 91.7 % (95% Cl 73%-99%) and 91.5% (95% Cl 80%-98%) sensitivity, 94% (95% Cl 90%-97%) and 82.4% (95% Cl 74%-89%) specificity, 99% (95% Cl 96%-99%) and 95% (95% Cl 89%-98%) negative predictive value and 63% (95% Cl 50%-74%) and 70.5% (95% Cl 61%-79%) positive predictive value, given malignancy rates 10% Bethesda III; 32% Bethesda IV. 45 unique combinations of genetic alterations were detected in the 96 operated ThyroSeq® v3 positive cases—34 combinations (76%) were present in <2 cases and only 3 combinations occurred 7 or more cases. Forty-six (48%) nodules had RAS mutations, either combined with other mutations 31% (23/30, 77% malignant) or alone, 17% (10/16, 63% malignant), followed by THADA/IGF2BP3 gene fusion changes,11.5% (10 /11 malignant). Three cases with BRAF mutations (1 V600E, 1 K601E, 1 K601N) were malignant. Copy numbers alterations alone were present in 10 (5/ 10 malignant) and the gene expression profile alone was positive in 5 cases (all malignant). Mutations that were associated with benign pathology were PTEN, DICER1, E1F1AX and TP53. There were 6 false negative cases, 5 low risk cancers by American Thyroid Association criteria and 1 NIFTP. Conclusion: The high BCR of Thyroseq® v3 for Bethesda III category avoids surgery for majority of patients. A more comprehensive mutational and fusion panel reveals the complexity of the genetic signature of indeterminate nodules. Future larger and likely multicenter studies will be required to define the associated cancer risk and potential prognosis associated with adjunct molecular testing.