Preoperative Diagnostic Categories of Noninvasive Follicular Thyroid Neoplasm with Papillary-Like Nuclear Features in Thyroid Core Needle Biopsy and Its Impact on Risk of Malignancy.

This study was designed to evaluate the preoperative diagnostic categories of noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) using thyroid core needle biopsy (CNB) and to analyze its impact on the risk of malignancy (ROM). A total of 2687 consecutive thyroid CNBs were reviewed retrospectively and classified into six diagnostic categories using a standardized reporting system similar to the Bethesda System for Reporting Thyroid Cytopathology. Diagnostic categories of CNBs were compared with the final surgical diagnoses, and the ROM in each category was calculated both before and after excluding NIFTP from malignancy. Of 946 surgically resected cases, 683 were diagnosed as papillary thyroid carcinoma (PTC), and 32 (4.7% of PTC) were reclassified as NIFTP. The CNB diagnostic categories of NIFTP were as follows: follicular neoplasm in 20 (62.5%; 14, with nuclear atypia), indeterminate lesion in 11 (34.4%), and suspicious for malignancy in one (3.1%). When combined, NIFTP and encapsulated follicular variant of PTC (EFVPTC) were more often categorized as follicular neoplasm compared with other PTC variants including infiltrative FVPTC. Exclusion of NIFTP from malignant diagnosis led to a significant decrease in the ROM in follicular neoplasm with nuclear atypia category. Thus, thyroid CNB enables to differentiate NIFTP/EFVPTC from other PTCs, providing a useful guide for optimal treatment in patients with these tumors.