Indeterminate thyroid nodules (TIR3A/TIR3B) according to the new Italian reporting system for thyroid cytology: A cytomorphological study

Objective The Italian reporting system for thyroid cytology classifies indeterminate lesions as TIR3A (low risk) or TIR3B (high risk) and is meant to provide practical guidance rather than a detailed consideration of morphological features. We aimed to assess which cytological features have the most diagnostic value and whether they are effective in classifying nodules as either TIR3A or TIR3B and in predicting histological outcomes. Methods Thyroid fine‐needle aspirates from 111 indeterminate nodules were reviewed blinded to clinical information, TIR3A/TIR3B classification, and histology in order to assess which cytological features (pooled into artefacts, smear background, architectural and nuclear atypia, and oncocytes) differentiate TIR3A from TIR3B, and benign from malignant histological outcomes. Results Of the cytological features examined, those specific for TIR3B included high cellularity, nuclear atypia, oncocyte predominance and transgressing vessels. Features specific for TIR3A included artefacts, low cellularity and oncocyte sparseness. Other features, such as microfollicules/trabeculae, were non‐specific. Due to the different distributions of these features, three TIR3B subgroups were identifiable: follicular lesions with oncocytic changes, pure follicular lesions, and follicular lesions with nuclear atypia, whereas no subgroups were identifiable in TIR3A. Nuclear atypia was a significant indicator of malignancy, whereas oncocyte predominance was not a reliable predictor of malignancy. High cellularity and microfollicules/trabeculae were not indicative of any histological outcome. Conclusions The majority of the assessed features were good predictors of histological outcomes. The TIR3A category included undefined nodules due to the absence of characterising features, whereas the TIR3B category included nodules with a greater number of distinguishing features. The cytological criteria for categorising thyroid aspirates as indeterminate are not equivalent across reporting systems. This study aims to ascertain the reliability of cytomorphological features associated with the Italian reporting system TIR3A and TIR3B categories. Determining the strengths and limitations of individual cytological criteria could help to improve the management of thyroid nodules, regardless of the reporting format.