Non-diagnostic thyroid nodules after application of the Bethesda system: a study evaluating the interval for repeat aspiration for non-diagnostic results

Background: The non-diagnostic category of the Bethesda system has a low malignancy rate; nevertheless, repeat ultrasonography-guided fine needle aspiration (US-FNA) is recommended with no consensus for follow-up intervals. Purpose: To investigate whether a six-month time interval for repeat US-FNA is appropriate for non-diagnostic thyroid nodules after applying the Bethesda system. Material and Methods: From January 2010 to January 2014, 322 thyroid nodules ≥10 mm with non-diagnostic results on initial US-FNA with any follow-up with US or US-FNA were included (mean age = 52.2 years ± 12.4). Clinical and US features were compared according to size change (increase, no change, decrease) and follow-up interval (<6 months and ≥6 months after initial US-FNA), and the minimal time interval observed in nodules with size increase on follow-up. The outcome of non-diagnostic nodules according to follow-up interval and size change were evaluated. Results: Of the 322 nodules, 13 (4.0%) were malignant and 309 (96.0%) were benign. None of the 82 nodules with decreased size were malignant and 90.2% nodules with decreased size were found after six months. Of 175 nodules with repeat US-FNA, repeat cytology results did not differ significantly between the groups with follow-up US-FNA before and after six months (P = 0.337). No significant differences were seen in size or extrathyroidal extension among the 13 malignant nodules according to the six-month interval (all P > 0.05), and no lateral lymph node metastasis was present. Conclusion: Repeat US-FNA for initially non-diagnostic thyroid nodules can be performed at a six-month interval after initial procedure without tumor progression.