Background: Patients with non-diagnostic thyroid fine needle aspiration cytology (FNAC) results undergo repeat FNAC or core needle biopsy (CNB) for definite diagnosis or surgical resection, or are followed up by clinical and ultrasound surveillance. We aimed at evaluating the risk of malignancy in patients with non-diagnostic FNACs and their clinical outcomes according to the follow-up modality.
Methods: We retrospectively reviewed 1,496 (8.8 %) cases with a non-diagnostic result on a first aspiration among 17,045 thyroid FNACs performed between October 2008 and August 2012. Of the non-diagnostic FNACs, 389 patients underwent a second FNAC; 125, CNB; and 89, thyroidectomy by clinical indication. The remaining patients were clinically followed up.
Results: The rate of a second non-diagnostic result was significantly higher on repeat FNAC than on CNB (33.2 vs. 2.4 %; p < 0.001). There was no significant difference in the malignancy risk among patients initially non-diagnostic, twice non-diagnostic, and thrice or more non-diagnostic, nor did this differ from the rate following CNB. No further malignancy was found in cases with ≥2 non-diagnostic CNBs. The malignancy risk was 51 % in those who underwent thyroidectomy. The sensitivity for detecting malignancy was 65 and 70 % for repeat FNACs and first CNBs, respectively, with no false positives seen in either test.
Conclusions: Approximately one-third of repeat FNACs after an initial non-diagnostic aspirate are non-diagnostic on repeat examination, and the malignancy risk may not reduce following repetitively non-diagnostic FNACs. However, a single CNB may be enough to exclude malignancy risk for patients with a non-diagnostic aspirate.