Impact of non-invasive follicular thyroid neoplasm with papillary-like nuclear features on the Bethesda system for reporting thyroid cytopathology: a multi-institutional study in five Asian countries

Several Western studies showed that the recent introduction of non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) significantly decreased risk of malignancy for cytological diagnostic categories. We aimed to determine the impact of NIFTP on risk of malignancy within a cohort of thyroid nodules from Asian countries, and to compare distribution of diagnostic categories between NIFTP and invasive encapsulated follicular variant of papillary thyroid carcinoma (eFV-PTC). Consecutive thyroid fine-needle aspirates from six institutions were retrospectively analysed. Histopathology slides with a diagnosis of eFV-PTC were reviewed and reclassified into invasive eFV-PTC and NIFTP. The risk of malignancy was calculated with and without NIFTP. Of 11,372 thyroid nodules, 2044 had available surgical follow-up. NIFTP was diagnosed in 59 cases, which constituted 2.9% of all excised nodules, and 5.3% of malignant nodules. Preoperative cytological diagnoses for NIFTP were non-diagnostic (10.2%), benign (18.6%), atypia of undetermined significance/follicular lesion of undetermined significance (22.0%), follicular neoplasm/suspicious for follicular neoplasm (FN/SFN) (32.2%), suspicious for malignancy (SM) (11.9%), and malignant (5.1%). The only category which showed a relative reduction in risk of malignancy after reclassification of more than 20%, was FN/SFN (24.4%). There was a significantly higher prevalence of benign cytology in NIFTP (p = 0.04) and SM/malignant in invasive eFV-PTC (p = 0.05). A majority of NIFTP cases were classified in indeterminate categories, which decreased the corresponding risk of malignancy. However, the magnitude of NIFTP impact was much lower than in the Western reports. Asian countries may not experience significant effects of NIFTP reclassification on the practice of thyroid cytopathology.