Pediatric follicular thyroid carcinoma: a clinicopathologic study treated at a single institution during a 30-year period.

PURPOSE: Pediatric follicular thyroid carcinoma (FTC) is an exceptionally rare entity, and its management has largely been extrapolated from adult protocols. Recent revisions of the WHO classification have redefined FTC subtypes, but pediatric studies based on the new criteria are lacking. This study aimed to evaluate the clinicopathological features of pediatric FTC using the updated WHO classification and to compare outcomes with pediatric papillary thyroid carcinoma (PTC). METHODS: Medical records of patients < 19 years who underwent thyroidectomy between 1989 and 2019 at Ito Hospital were reviewed. Pathological diagnosis was reassessed according to the most recent WHO classification. Twenty-four FTC and 134 PTC cases were identified. Clinical characteristics, histological findings, and outcomes were analyzed. RESULTS: Pediatric FTC accounted for 15.2% of pediatric DTC, a significantly higher proportion than in adults (5.9%). Median age was 16 years; female-to-male ratio was 7:1. Subtypes were minimally invasive (25%), encapsulated angioinvasive (46%), and widely invasive (29%). Extensive vascular invasion (VI ≥ 4) was observed in 7 patients (29%). Solid, trabecular, or insular growth pattern were observed in 21 patients (87.5%). VI was associated with distant metastasis, but no patients experienced disease-specific death during median follow-up of 6.2 years. In contrast, PTC patients showed significantly higher recurrence rates (p < 0.05). CONCLUSION: Pediatric FTC demonstrates excellent prognosis, but its pathological spectrum differs from previous reports under older classifications. VI remains the key determinant of metastatic risk. These findings suggest the need for risk-adapted strategies tailored to pediatric FTC, with avoidance of overtreatment in low-risk cases.