Application of Strict Criteria for Noninvasive Follicular Thyroid Neoplasm with Papillary-Like Nuclear Features and Encapsulated Follicular Variant Papillary Thyroid Carcinoma: a Retrospective Study of 50 Tumors Previously Diagnosed as Follicular Variant PTC.

Noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) was recently proposed as a designation for a subset of follicular variant papillary thyroid carcinoma (FVPTC). Encapsulated FVPTC has been shown to be a fairly indolent tumor, and NIFTP are expected to represent the most indolent subset of these tumors. Many of the exclusion criteria for NIFTP related to architecture and a lack of psammoma bodies are designed to preclude the inclusion of more aggressive non-FVPTC tumors in this indolent group and also exclude the diagnosis of FVPTC. In addition to strict application of histologic features to ensure that NIFTP represents a subset of encapsulated FVPTC without invasion, other exclusion criteria including high mitotic activity and necrosis may also lead to a lack of one-to-one correlation between the diagnosis of NIFTP and encapsulated FVPTC without invasion. In this series, 50 cases previously diagnosed as FVPTC over a 2-year period from a large academic center are retrospectively reviewed for reclassification as NIFTP. Additionally, cases not meeting criteria for NIFTP are more accurately classified using the most up to date WHO criteria. Prior BRAF V600E mutation testing was examined for these tumors when available. Seventeen of 50 (34%) tumors met criteria for classification as NIFTP and, 17 (34%) were classified as encapsulated FVPTC with invasion. Strict application of architectural features led to classification of 12 (24%) tumors as non-FVPTC with a variety of more aggressive designations. Tumors classified as NIFTP and encapsulated FVPTC with invasion lacked lymph node metastases (0/4; 0/7, respectively) and BRAF mutations (0/12; 0/13, respectively). In contrast, infiltrative FVPTC, encapsulated PTC with or without invasion, and conventional PTC showed more aggressive features with lymph node metastases and BRAF V600E mutations. One case not meeting criteria for NIFTP maintained the diagnosis of encapsulated FVPTC without invasion but demonstrated significant mitotic activity (three mitoses/ten HPF) and lacked lymph node metastases and BRAF V600E mutation. These findings demonstrate the importance of using strict criteria, especially the lack of true papillary architecture, for the diagnosis of NIFTP and encapsulated FVPTC to ensure that only truly indolent tumors will be included in these diagnoses and to allow tumors with potential for more aggressive behavior to be appropriately treated.