ΔTg Assesses Radioiodine Treatment Response and Predicts Prognosis in Pediatric Differentiated Thyroid Cancer

Purpose: This study aims to define the relationship between the thyroglobulin change rate (ΔTg) after radioactive iodine (RAI) therapy and treatment response, determine its optimal threshold, and validate its prognostic value in pediatric patients with differentiated thyroid cancer (DTC). Methods: This retrospective study analyzed 123 RAI courses in 57 pediatric DTC patients with postoperative persistent disease. Treatment response was assessed at 6 months. Optimal ΔTg thresholds were determined using ROC analysis with bootstrap validation. Prognostic value for no evidence of disease (NED) and progression-free survival (PFS) was evaluated. Results: ΔTg significantly differed among response categories ( p <0.001): Complete Response (CR): 84.5%, Partial Response (PR): 53.8%, Stable Disease (SD): 4.9%, Progressive Disease (PD): -69.0%. ROC analysis defined optimal ΔTg thresholds: ΔTg ≥32% for Objective Response (OR=CR+PR; AUC=0.96, p <0.001), ΔTg ≤-14% for PD (AUC=0.93, p <0.001); patients with -14%<ΔTg<32% were classified as SD. Longitudinally, initial ΔTg ≥32% (n=29) vs. ΔTg ≤-14% (n=17) showed superior PFS (133.5 vs. 13.9 months, p <0.001) and higher NED rates (55.2% vs. 23.5%, p >0.05). The intermediate group (-14%<ΔTg<32%, n=11) exhibited outcome dissociation: NED rate (18.2%) aligned with ΔTg ≤-14% ( p >0.05), while PFS (126.4 months) resembled ΔTg ≥32% ( p >0.05). Multivariable analysis confirmed ΔTg ≤-14% predicted an 11.7-fold higher progression hazard (HR=11.70, 95%CI:3.27-41.89; p <0.001). Conclusions: This study establishes validated ΔTg thresholds for response assessment in pediatric DTC (OR: ≥32%; PD: ≤-14%; SD: -14%<ΔTg<32%). Initial ΔTg is a powerful independent prognostic factor. These evidence-based thresholds provide actionable guidance for personalized pediatric DTC management.