Risk-adapted trajectory selection for ultrasound-guided thyroid fine-needle aspiration biopsy: a multicenter study of in-plane and out-of-plane approaches.

BACKGROUND: Ultrasound-guided fine-needle aspiration biopsy (FNAB) is widely used for thyroid nodule evaluation, but nondiagnostic cytology can lead to repeat procedures and delayed management. Because needle approach is influenced by nodule size, anatomical risk, access route, and needle-tip visibility, simple in-plane versus out-of-plane comparisons may not fully reflect clinical decision-making. This study evaluated the association between needle approach and diagnostic adequacy and explored whether the findings could inform a preliminary, risk-adapted trajectory-selection concept.

METHODS: This multicenter retrospective cohort study included 506 thyroid nodules that underwent clinically indicated ultrasound-guided FNAB between September 2024 and December 2025 at three tertiary hospitals. One center formed the primary cohort, and two independent centers formed the external validation cohort. Diagnostic adequacy was defined as Bethesda category II-VI cytology, and Bethesda category I cytology was retained in the analysis as a nondiagnostic outcome rather than excluded. Multivariable logistic regression was used to evaluate the association between needle approach and diagnostic adequacy, with adjustment for nodule size category, complex anatomical location, operator experience, and center source in pooled analyses. Stabilized inverse probability of treatment weighting was performed as a sensitivity analysis.

RESULTS: Among 506 nodules, 289 were sampled using the in-plane approach and 217 using the out-of-plane approach. Diagnostic adequacy was higher with the in-plane approach than with the out-of-plane approach (98.3% vs. 85.7%; P < 0.001). The adjusted association remained significant in the primary cohort (adjusted odds ratio [OR], 9.39; 95% confidence interval [CI], 2.37-37.18), external validation cohort (adjusted OR, 14.51; 95% CI, 2.82-74.74), and pooled center-adjusted model (adjusted OR, 12.10; 95% CI, 4.28-34.20). Weighted sensitivity analysis yielded similar results. Satisfactory needle-tip visualization was more frequent with the in-plane approach (97.2% vs. 63.6%; P < 0.001), whereas immediate hematoma did not differ significantly between approaches (6.2% vs. 9.2%; P = 0.234).

CONCLUSION: The in-plane approach was associated with higher diagnostic adequacy without a statistically significant increase in immediate hematoma. These findings provide preliminary, hypothesis-generating multicenter evidence that may inform risk-adapted trajectory selection for ultrasound-guided thyroid FNAB.