Thyroid cytology: practical tricks and pitfalls

Thyroid lesions are a common finding, especially in the adult population, based on the evidence that more than 50% of individuals have thyroid nodules. The increasing detection of these lesions is mostly due to frequent ultrasonographic head and neck evaluation, which can now identify small subcentimeter nodules. Fortunately, most of these nodules are benign (70%), with only 5-10% of them attributed as malignant lesions. However, the remaining 20% falling into the category of indeterminate lesions which can lead to some pitfalls and tricky evaluations. Since 1996, different classification systems have been introduced and among them, the most worldwide adopted is the Bethesda System for Reporting Thyroid Cytopathology (TBSRTC). It is well-known that TBSRTC as well as other classification systems, subclassified indeterminate lesions into subgroups which specifically for the TBSRTC include a) atypia of undetermined significance (AUS), b) follicular or oncocytic cell neoplasm (FN) and c) suspicious for malignancy (SFM). However, despite the high positive predictive value (97%-99%), sensitivity (65%-99%) and specificity (72%-100%) of thyroid FNAC, diagnostic pitfalls exist that can lead to false positive and/or false negative results. This inconvenience is mostly due to the overlapping of morphological features in terms of cells and even background. This review discusses the most important potential pitfalls in the cytologic evaluation of thyroid lesions that can lead to such diagnostic errors.