Risk stratification of cytologically indeterminate thyroid nodules with nondiagnostic or benign cytology on repeat FNA: Implications for molecular testing and surveillance.

BACKGROUND: Evidence guiding the management of cytologically indeterminate thyroid nodules with nondiagnostic (ND) or benign cytology on repeat fine-needle aspiration (FNA) is limited. This study evaluates the utility of molecular testing and estimates the risk of noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) and cancer among such nodules.

METHODS: This was a retrospective single-institution review of thyroid nodules from adults that were classified as atypia of undetermined significance (AUS) or follicular neoplasm (FN) on initial FNA and underwent repeat FNA for cytology and Afirma testing (June 2013-July 2021). The association between repeat FNA cytology and RNA yield for Afirma was determined. Histologic outcomes were integrated with Afirma results to define end points for each nodule.

RESULTS: A total of 691 AUS and FN nodules underwent repeat FNA and Afirma testing. Diagnostic Afirma results were obtained in 98% of cases overall and in 91% of nodules with ND cytology on repeat FNA. Using combined molecular and histologic end points, the NIFTP and/or cancer prevalence for nodules with ND cytology on repeat FNA was 9% (95% confidence interval [CI], 0.042-0.182), falling between those nodules classified as benign (5%; 95% CI, 0.029-0.094) and those classified as AUS or FN (18%; 95% CI, 0.140-0.218) on repeat FNA, although not reaching statistical significance from either subgroup (p = .38 and .10, respectively).

CONCLUSIONS: AUS and FN nodules that are ND on repeat FNA have low but nonnegligible risk of NIFTP and/or cancer and may benefit from molecular testing, given the low test failure rate in this subgroup. Conversely, AUS and FN nodules reclassified as benign on repeat FNA have a very low risk of NIFTP and/or cancer and are unlikely to benefit from molecular testing.