Delayed initial radioiodine therapy related to incomplete response in low‐ to intermediate‐risk differentiated thyroid cancer

Summary Objective Whether the initiating time of radioiodine ( RAI ) therapy will affect the clinical outcome in differentiated thyroid cancer ( DTC ) remains controversial. The objective of this study was to evaluate the impact of RAI therapy initiating time on response to initial therapy in low‐ to intermediate‐risk DTC . Methods A total of 235 consecutive patients with low‐ to intermediate‐risk DTC were retrospectively reviewed. According to the time interval between thyroidectomy and RAI therapy, patients were divided into Group 1 (interval < 3 months, n = 187) and Group 2 (interval ≥ 3 months, n = 48). Response to RAI therapy was evaluated as excellent, indeterminate, biochemical incomplete or structural incomplete response ( ER , IDR , BIR or SIR ) with a median follow‐up of 780 days. The univariate and multivariate analyses were further conducted to identify factors associated with incomplete response ( IR , including BIR and SIR ). Results Response to initial therapy was significantly different between 2 groups ( P  <   .05), after excluding the impact of other risk factors (age, gender, histological type, status of T and N, RAI dose, thyrotropin, stimulated thyroglobulin and follow‐up time). A significantly higher IR rate (18.8% vs 4.3%, P  =   .001) and a lower ER proportion (62.5% vs 78.1%, P  =   .027) were observed in Group 2. By univariate analysis, both T status and N status, stimulated thyroglobulin and time interval were significant risk factors for IR ( P  <   .05). Multivariate analysis demonstrated that the time interval was an independent risk factor for IR ( P  =   .008). Conclusions Delayed initial RAI therapy (≥3 months after thyroidectomy) related to incomplete response in low‐ to intermediate‐risk DTC .