Atypia of Undetermined Significance in Thyroid Fine Needle Aspirates: a 4-Year Audit of Thy3a Reporting.

Objective: Thyroid nodules are common within the general population. Cytological analysis of fine needle aspirates (FNAs) of these lesions allows for identification of those that require further surgery. A numerical classification system is in place to streamline reporting. The 3a category is used for lesions that are neither benign nor malignant but show atypia of undetermined significance. We reviewed our use and clinical outcomes of Thy3a over a 4-year period.

Methods: All thyroid FNAs performed at this institute from January 2012 to December 2015 were identified from our laboratory information system using SNOMED codes. Cytology was correlated with histology.

Results: Of the 1,259 FNAs reported at this institute, Thy3a constituted only 1.2% ( = 16) of all cases, with a malignancy rate of 7%. Five Thy3a cases had a repeat FNA that was reported as Thy2 (benign), 1 as Thy1c (cyst), 1 as Thy3f (follicular lesion), and 1 as Thy5 (malignant). Six cases without repeat FNA were follicular adenomas at resection. Two cases were lost to follow-up. Within all thyroid cytology categories in this 4-year period, we had a false-positive rate of 1.9% and a false-negative rate of 0.3%.

Conclusions: The Thy3a subclassification has varied diagnostic criteria and lacks reproducibility. Despite the rare use of the Thy3a category at our centre, our diagnostic accuracy remained high. At this time, further Thy3a cohort studies are required to assess the real benefits of this category.