Clinicopathologic Characteristics of Pediatric Follicular Variant of Papillary Thyroid Carcinoma Subtypes: A Retrospective Cohort Study.

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Follicular patterned thyroid nodules with nuclear features of papillary thyroid carcinoma encompass a range of diagnostic categories with varying risks of metastatic behavior. Subtypes include invasive encapsulated fvPTC (Ienc-fvPTC) and infiltrative fvPTC (inf-fvPTC) with tumors lacking invasive features classified as non-invasive follicular thyroid neoplasm with papillary-like features (NIFTP). This study aimed to report the clinical and histologic features of pediatric cases meeting criteria for these histological subtypes with specific focus on Ienc-fvPTC and inf-fvPTC. In this retrospective cohort study, pediatric patients with thyroid neoplasms showing follicular patterned growth and nuclear features of papillary thyroid carcinoma noted on surgical pathology between January 2010 and January 2021 were retrospectively reviewed and classified according to the recent 2022 World Health Organization (WHO) criteria. Clinical and histopathologic parameters were described for NIFTP, Ienc-fvPTC, and inf-fvPTC subtypes, with specific comparison of Ienc-fvPTC and inf-fvPTC cases. The case cohort included 42 pediatric patients, with 6 (14%), 25 (60%), and 11 (26%) patients meeting criteria for NIFTP, Ienc-fvPTC, and inf-fvPTC, respectively. All cases were re-reviewed, and 5 patients originally diagnosed with Ienc-fvPTC prior to 2017 were reappraised as NIFTP. The NIFTP cases were encapsulated tumors without invasive features, lymph node or distant metastasis, or disease recurrence. Ienc-fvPTC tumors demonstrated clearly demarcated tumor capsules and capsular/vascular invasion, while inf-fvPTC tumors displayed infiltrative growth lacking a capsule. Inf-fvPTC cases had increased prevalence of malignant pre-operative cytology, lymph node metastasis, and distant metastasis ( < 0.01). These cases were treated with total thyroidectomy, lymph node dissection, and subsequent radioactive iodine therapy. Preliminary genetic findings suggest a predominance of fusions in inf-fvPTC cases versus point mutations in Ienc-fvPTC ( < 0.01). Pediatric NIFTP and fvPTC subtypes appear to demonstrate alignment between clinical and histological risk stratification, with indolent behavior in Ienc-fvPTC and invasive features in inf-fvPTC tumors.



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