Risk Stratification of Thyroid Nodules Diagnosed as Follicular Neoplasm on Core Needle Biopsy.

Background: This study assessed risk stratification and diagnostic performance for malignancy in thyroid nodules diagnosed as follicular neoplasm (FN) based on core needle biopsy (CNB) subcategories.

Methods: A total of 313 consecutive nodules (>1 cm) diagnosed as FN on CNB with corresponding surgical histology were included. FN subcategories were classified retrospectively for nodules diagnosed before 2022 (retrospective dataset) and prospectively for nodules diagnosed since 2022 (prospective dataset). CNB subcategories were determined using histologic criteria based on architectural uniformity and nuclear atypia, as modified from the 2019 Korean CNB pathology guideline. The diagnostic performance of CNB subcategories, nodule size, and ultrasound risk stratification systems (RSSs) for malignancy was assessed.

Results: CNB subcategory IVb showed a significantly higher malignancy risk compared to other subcategories in both datasets (34.5%-83.7% vs. 4.2%-13.6%, P<0.001). It was also identified as an independent predictor of malignancy in both datasets (P< 0.001), whereas nodule size and all ultrasound RSSs were not predictive of malignancy, including noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) (P≥0.079). CNB subcategory IVb demonstrated higher sensitivity for malignancy and a lower surgical rate for benign nodules compared to the nodule size criterion (>2 cm). The combined criterion of CNB subcategory IVb or nodule size >3 cm identified all malignant tumors, excluding NIFTP, in the prospective dataset.

Conclusion: CNB subcategory IVb effectively stratifies malignancy risk in thyroid nodules and outperforms nodule size (>2 cm) and ultrasound RSSs in diagnostic performance. Non-IVb nodules ≤3 cm can be safely managed with ultrasound surveillance instead of immediate surgery.