Pediatric Differentiated Thyroid Carcinoma of Follicular Cell Origin: Prognostic Significance of Histologic Subtypes.

BACKGROUND Thyroid cancers are rare in the pediatric age group, and unlike in adults, few data are available regarding the clinical implication of histologic subtypes in the pediatric population. The purpose of the current study was to determine the prognostic significance of histologic subtypes of differentiated thyroid cancer (DTC) in a large series of children and adolescents followed at a single institution. METHODS A retrospective review was conducted of all pediatric DTC patients who were treated and followed between 1988 and 2012. Sixty-two patients (median age at diagnosis 13.8 years, median age at follow-up 18 years, 77% female) were assessed. The most common subtypes included classic papillary thyroid carcinoma (PTC; 48%), diffuse sclerosing PTC (16%), and follicular variant PTC (15%); 37% were considered "high-risk" histologies based on adult criteria. RESULTS In a multivariate model, only extensive extrathyroidal extension (ETE), defined as the presence of two or more microscopic foci of tumor cells ≤1 mm in size each or any foci >1 mm in size invading beyond the thyroid capsule into perithyroid soft tissue or organs, was significantly associated with extent of disease at presentation. At last follow-up, 76% of subjects had no evidence of disease, 18% had persistent disease, and 5% had recurrent/progressive disease. Event-free survival was associated with extent of disease at presentation (p = 0.01), extensive ETE at diagnosis (p < 0.01), and male sex (p = 0.01), but not histologic subtype (p = 0.20). CONCLUSIONS Pediatric DTC carries an excellent prognosis. Extensive ETE at diagnosis was found to be an independent predictor of extent of disease at presentation, as well as event-free survival. Unlike in the adult population, "high-risk" histologic subtypes did not independently predict extent of disease at presentation or event-free survival in this pediatric population with DTC.