Impact of Co-Mutations and Genetic Variations on Malignancy Risk in RAS-Positive Indeterminate Thyroid Nodules: an Institutional Experience.

Mutations in RAS proto-oncogenes (NRAS, HRAS, KRAS) are common in thyroid nodules, though their prognostic significance remains unclear. This retrospective study analyzed 354 thyroid nodules from 346 patients (2018-2023) to investigate the clinical and pathological implications of isolated RAS mutations and RAS with co-occurring genetic alterations. Isolated RAS mutations were found in 41.0% (n = 145), while 54.8% (n = 194) had RAS with additional molecular alterations; NRAS was the most frequent subtype (62.1%). Among co-occurring mutations, EIF1AX (46.7%) and TERT (26.7%) were the most common, primarily in NRAS-positive cases. Surgical follow-up data from 302 cases revealed a malignancy rate of 52.3% (n = 158), with 60.1% (n = 95) being invasive encapsulated follicular variant of papillary thyroid carcinoma (IEFVPTC). NRAS mutations appeared in 64.6% of malignant cases. Isolated RAS mutations were mainly associated with benign/low-risk neoplasms (47.0%), notably follicular adenomas and encapsulated non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP), or malignancies (41.0%). The malignancy rate was higher in nodules with a RAS mutation plus one concomitant molecular alteration (54.3%), and nearly 100% in those with three additional genetic alterations. Co-occurring genetic alterations with RAS mutations markedly increased the risk of malignancy compared with isolated RAS mutations (Fisher's exact test, two-tailed p = 0.0026) and were associated with more aggressive tumor phenotypes, whereas isolated RAS mutations were more common in indolent neoplasms. Comprehensive molecular profiling is essential for accurate risk stratification and management of indeterminate thyroid nodules.