Ultrasound Follow-Up of Benign Thyroid Nodules: A Scoping Review

Background: For cytologically benign thyroid nodules with very low to intermediate suspicion ultrasound patterns, optimal ultrasound follow-up intervals and outcomes of discontinuing follow-up are unclear. Methods: Ovid MEDLINE, Embase, and Cochrane Central were searched through August 2022 for studies comparing different ultrasound follow-up intervals and discontinuation versus continuation of ultrasound follow-up. The population was patients with cytologically benign thyroid nodules and very low to intermediate suspicion ultrasound patterns, and the primary outcome was missed thyroid cancers. Utilizing a scoping approach, we also included studies that were not restricted to very low to intermediate suspicion ultrasound patterns or evaluated additional outcomes such as thyroid cancer-related mortality rate, nodule growth, and subsequent procedures. Quality assessment was performed, and evidence was synthesized qualitatively. Results: One retrospective cohort study (n = 1254; 1819 nodules) compared different first follow-up ultrasound intervals for cytologically benign thyroid nodules. There was no difference between >4- versus 1- to 2-year intervals to first follow-up ultrasound in the likelihood of malignancy (0.4% [1/223] vs. 0.3% [2/715]), and no cancer-related deaths occurred. Follow-up ultrasound at >4 years was associated with increased likelihood of ≥50% nodule growth (35.0% [78/223] vs. 15.1% [108/715]), repeat fine needle aspiration (19.3% [43/223] vs. 5.6% [40/715]), and thyroidectomy (4.0% [9/223] vs. 0.8% [6/715]). The study did not describe ultrasound patterns or control for confounders, and analyses were based on interval to first follow-up ultrasound only. Other methodological limitations were not controlling for variability in follow-up duration and unclear attrition. The certainty of evidence was very low. No study compared discontinuation of ultrasound follow-up versus continuation. Conclusions: This scoping review found that evidence comparing different ultrasound follow-up intervals in patients with benign thyroid nodules is limited to one observational study, but suggests that the subsequent development of thyroid malignancies is very uncommon regardless of follow-up interval. Longer follow-up may be associated with more repeat biopsies and thyroidectomies, which could be related to more interval nodule growth that meets thresholds for further evaluation. Research is needed to clarify optimal ultrasound follow-up intervals for low to intermediate suspicion cytologically benign thyroid nodules and outcomes of discontinuing ultrasound follow-up for very low suspicion nodules.