SAT-461 NIFTP: A Painstaking Diagnosis Through the Pathologist’s Eyes

Non-invasive encapsulated follicular variant of papillary thyroid cancer (EFVPTC) was recently reclassified as non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFT-P).(1) In 2018, revised and stricter criteria were proposed for a lesion to qualify as NIFT-P including no well-formed papilla or BRAF-V600E mutation.(2) We are presenting an interesting case to highlight the importance of scrutinizing pathology slides to diagnose NIFTP with these more strict criteria. 35-year-old female from Puerto-Rico was diagnosed with Graves’ disease. After 2 years of methimazole treatment, total thyroidectomy was planned for definitive treatment of Graves’ disease. During the work up, she was noted to have a cystic nodule in isthmus, a 1.1 cm hypoechoic nodule in left mid-lobe and a 1.1 cm isoechoic rounded mass in left level III neck, which was initially thought to be a lateral aberrant thyroid remnant. Her thyroid uptake scan was consistent with a multinodular goiter with no uptake in the extrathyroidal mass. The mass was biopsied and showed Atypia of Undetermined Significance (AUS) Bethesda III with washout positive for thyroglobulin (Tg). Total thyroidectomy with bilateral central and left lateral neck dissection was performed. The pathology showed an intrathyroidal 1.2 cm EFVPTC with predominant follicular features and <1% papillae, without tumor capsular invasion. The initial diagnosis was NIFT-P with a background of chronic thyroiditis. However, on pathology, the level III neck mass was a 2 cm metastatic node with classical PTC. ThyroSeq mutational analysis of tissue blocks for both the thyroid nodule and lymph node were positive for NCOA4-RET (RET-PTC3) gene fusion, a BRAF-V600E-like mutation found in classical PTC. On review of her pathology, the thyroid lesion was noted to have more than one papilla, though <1% papillae and was >30% solid, hence not qualifying as NIFT-P and her histological diagnosis was changed to EFVPTC. She was staged as AJCC 8(th) edition stage 1 with intermediate ATA risk for which she received adjuvant therapy of 101 mCi (131)I. Although classification into NIFT-P has been shown to reduce overtreatment of low risk encapsulated PTC, pathology slides should be closely scrutinized to ensure fulfillment of all criteria in order for a lesion to qualify as NIFT-P. This will minimize failure to recognize PTCs, that would warrant closer follow up and surveillance for recurrence. 1. Rossi, Esther D, et al. Noninvasive Follicular Thyroid Neoplasm with Papillary-Like Nuclear Features: Update and Diagnostic Considerations—a Review. Endocrine Pathology 30.2 (2019) 2. Nikiforov, Yuri E et al. Change in Diagnostic Criteria for Noninvasive Follicular Thyroid Neoplasm With Papillary-like Nuclear Features. JAMA oncology vol. 4,8 (2018)