Differences in Risk of Malignancy and Management Recommendations in Subcategories of Thyroid Nodules with Atypia of Undetermined Significance or Follicular Lesion of Undetermined Significance: The Role of Ultrasound-Guided Core-Needle Biopsy

Background: The cytopathologic description of atypia of undetermined significance (AUS)/follicular lesion of undetermined significance (FLUS) includes nine different criteria in The Bethesda System, and the risk of malignancy in this category shows a wide range. The objectives of the present study were to determine whether ultrasound (US)-guided core-needle biopsy (CNB) indicates a different malignant risk, and to identify management recommendations, malignant US findings, and distribution of CNB readings in subcategories of AUS/FLUS category, as seen on previous thyroid fine-needle aspiration readings. Methods: From October 2008 to July 2011, 191 thyroid nodules of 191 patients who had previously been diagnosed with nuclear atypia (Group AUS; n=84) and microfollicular architecture (Group FLUS; n=107) were enrolled in our retrospective study. Final diagnoses were obtained in 142 nodules after surgery and clinico-radiological follow-up. We compared the malignancy risk, management recommendation, malignant US findings, and distribution of CNB readings between the two groups and calculated the diagnostic value of CNB. Results: With CNB, the final malignancy results were greater in Group AUS (65%, 33/51) than Group FLUS (14.3%, 13/91; p<0.001), and there were more surgical candidates in Group AUS (57.8%, 46/84) than Group FLUS (19.6%, 21/107; p<0.001). CNB showed 95.8% diagnostic accuracy for identifying malignancies and 19.4% inconclusive readings. Malignant US findings were seen more frequently in Group AUS (76.5%, 39/51) than Group FLUS (52.7%, 48/91; p=0.007). Malignant CNB readings were statistically more frequent in Group AUS (49.2%, 41/84) than Group FLUS (9.4%, 10/107; p<0.001), and benign readings were statistically more frequent in Group FLUS (58.9%, 63/107) than Group AUS (28.6%, 24/84; p<0.001). Conclusions: US-guided CNB demonstrated that Group AUS showed a higher risk of malignancy, of becoming surgical candidates, of having malignant US findings, and of having malignant CNB readings than Group FLUS. Further management guidelines for Group AUS should differ from Group FLUS.