Distinct molecular profiles of indeterminate and malignant thyroid nodules in patients under 21 years of age.

Although uncommon, thyroid nodules (TN) in pediatric and young adult patients carry higher malignancy risk and often present with a high burden of metastatic disease than adults. The molecular features underlying this distinct clinical behavior remain unclear. We analyzed Afirma Genomic Sequencing Classifier (GSC) data from 283,621 TN, comparing patients <21 and ≥21 years. Cytology (Bethesda), GSC benign (B) vs suspicious (S) calls, and Afirma Xpression Atlas (XA) variant/fusion profiles were evaluated in GSC-S and Bethesda V/VI samples. Genome-wide expression was used to derive pathway signatures and thyroid cancer-related scores: BRAF-RAS score (BRS), ERK, follicular and epithelial-to-mesenchymal transition (FMT, EMT) and thyroid differentiation scores (TDS). Among 2,397 patients <21 (median age 18.9; 81.4% female) and 281,224 adults ≥21 (median age 59.8; 77.1% female), <21 samples showed more Bethesda V/VI cytology (14.5% vs 5.0%; p<0.0001) and a lower GSC-B rate (43.5% vs 68.8%; p<0.0001). In GSC-S samples, total variant detection was higher in <21 (45.3% vs 37.4%), with enriched BRAF p.V600E, TSHR, and DICER1 variants, while HRAS variants were more common in adults (all p<0.01). Gene fusions involving RET, NTRK3 and ALK were enriched in <21 (14.5% vs 5.5%; p<0.0001). TERT promoter mutations were absent in <21 yrs GSC-S and Bethesda V/VI samples (vs 4.2% and 9.3% in adults). GSC-S <21 showed cell-cycle pathway enrichment. RET/NTRK/ALK-positive <21 demonstrated enrichment of angiogenesis and EMT pathways, higher ERK/EMT/FMT scores, and lower BRS/TDS scores vs genotyped-matched adults. These molecular differences provide mechanistic insight into the more invasive phenotype in pediatric and young adult TN.