Background: Ultrasound (US)-guided fine needle aspiration cytology (FNAC) is an accurate, reliable, and simple method to identify a thyroid nodule as benign or malignant. However, non-diagnostic cytology results for thyroid nodules are a major limitation of US-guided FNAC.
Purpose: To investigate the incidence of thyroid cancer among cases with non-diagnostic results on FNAC and to provide suggestions for the management of thyroid nodules that are initially non-diagnostic by FNAC according to ultrasonographic findings.
Material and methods: From July 2006 to December 2009, 10,317 thyroid nodules in 6684 consecutive patients underwent US-guided FNAC at our institute. Among these, 871 thyroid nodules (8.4%) were diagnosed as non-diagnostic on initial cytologic evaluation and 196 underwent a second or third FNAC. Twenty-seven thyroid nodules (18.9%) underwent surgery, while 116 thyroid nodules were cytologically confirmed as benign with no remarkable change on follow-up US were included. We retrospectively reviewed the US findings for a total of 143 thyroid nodules (123 benign nodules and 20 malignant nodules). The US features that we compared included composition, echogenicity, margin, calcifications, shape, and underlying echogenicity.
Results: In total, thyroid cancer was diagnosed in 20 nodules (14.0%). The size of the nodule was significantly associated with malignancy (P < 0.05). Most of the sonographically probable benign nodules were found to be benign (97.6%). Suspicious nodules on US were thyroid cancer in 43.2% of cases. Marked hypoechogenicity, microlobulated or irregular margin, microcalcifications, and taller-than-wide shape were significant US findings that correlated with malignancy (P < 0.05). The diagnostic performance of ultrasound for initially non-diagnostic thyroid nodules was as follows: sensitivity of 90.0%, specificity of 65.0%, positive predictive value of 29.5%, and negative predictive value of 97.6%.
Conclusion: In terms of management of thyroid nodules with non-diagnostic FNAC cytology, US evaluation is a feasible and useful method for predicting malignancy.