Thyroid Nodules with Indeterminate Cytology and Negative Molecular Profile: Prevalence of Malignancy and Practice Paradigms for Surveillance

Background: In the era of molecular testing, thyroid nodules with indeterminate cytology are increasingly being managed nonoperatively. The false-negative rates of these molecular tests, and therefore missed malignancies, are not well defined in real-world clinical practice. Methods: This retrospective study of patients undergoing fine needle aspiration (FNA) biopsy at our health system between November 2017 and March 2022 included nodules with The Bethesda System for Reporting Thyroid Cytopathology (TBSRTC) III and IV cytology and negative, currently negative, or negative but limited ThyroSeq version 3 (TSv3) results. Surgical pathology of resected nodules, as well as details of ultrasound (US) surveillance practices, was recorded. A range of prevalence of malignancy (PoM) estimates were calculated based on all nodules (PoM low) and surgically resected nodules (PoM high). Results: The study cohort consisted of 556 nodules. TSv3 results were distributed as 443 (80%) negative, 85 (15%) currently negative, and 28 (5%) negative but limited. Overall, 75 nodules were resected: 54 nodules (9.7%) had immediate surgery, and 21 nodules (3.8%) had delayed surgery after surveillance imaging. Currently negative and negative but limited nodules were more likely to undergo immediate surgical resection compared with negative nodules (20%, 18%, and 7%, respectively, p < 0.001). The PoM in molecularly benign TBSRTC III and IV thyroid nodules ranged between 3% and 23% depending on the inclusion of all versus resected nodules. TBSRTC IV molecularly benign nodules had a higher PoM than TBSRTC III (PoM low 7.3% vs. 1.6%, p < 0.001; PoM high 48% vs. 13%, p = 0.0013). In the 90% of nodules that were managed nonoperatively, 63% had at least one surveillance US. Timing of initial surveillance US ranged from 3 to 60 months (median 13 months, interquartile [IQR] 11-19 months). Median follow-up duration was 25 months (IQR 17-34 months). Nodule growth occurred in 24% of nodules; only a minority (7%) underwent repeat FNA. Conclusions: Negative subtype of TSv3 should be considered in clinical management recommendations. For negative but limited samples, repeat FNA should be performed. Optimal surveillance strategy for nonresected negative and currently negative nodules remains unknown. Until further real-world data are available, surveillance ultrasonography is recommended for TSN and TSCN nodules, similar to the ATA guidelines for TBSRTC II nodules.