'Indeterminate for malignancy' (Tir3/Thy3 in the Italian and British systems for classification) thyroid fine needle aspiration (FNA) cytology reporting: Morphological criteria and clinical impact

Background: The British system (Thy1-5), the Bethesda system for reporting thyroid cytopathology (BSRTC) and the Italian Society of Anatomic Pathology and Cytology (SIAPEC) classification represent the most important international classifications for thyroid cytopathology. Irrespective of the system used, the 'indeterminate' categories are still debated among cytopathologists, particularly with regard to diagnostic criteria, clinical impact of subclassification and role of molecular techniques. Aim: We aimed to find answers to the following questions: Are there shared criteria in cytological preparations that allow the division of indeterminate follicular lesions into subcategories? What is the true clinical impact of this possible subclassification? Methods: Among 1150 consecutive thyroid fine needle aspiration (FNA) specimens, 80 patients had nodules with a final cytological report of Tir3 (SIAPEC)/Thy3. These 80 cases were re-evaluated and subclassified according to morphological criteria into three groups: pure follicular proliferations, Hürthle cell follicular lesions and atypical proliferations. Results: Sixteen (20%) cases were categorized as pure follicular proliferations, 40 (50%) as Hürthle cell follicular lesions and 24 (30%) as atypical proliferations. Surgery was performed in 57 cases (71%). Cyto-histological correlation showed that follicular adenoma was the most frequent final diagnosis in the cases treated by surgery (24/57, 42%). The overall malignancy rate in the Tir3 category was 28% (16/57). Atypical proliferations were more often malignant than either of the follicular groups (53% versus 19%, P = 0.019). Conclusions: A five-tiered classification, subdividing the 'indeterminate for malignancy' class into 'follicular proliferations' and 'atypical lesions' could be adopted. As a result of their higher risk of malignancy, surgical management of the atypical lesions would be justified. In future, the introduction of a genetic panel might contribute to their stratification, to the determination of a more accurate risk of malignancy of the atypical lesions and to the verification of follicular proliferations that are benign.