Efficacy assessment of repeat iodine-131 treatment in patients with papillary thyroid carcinoma assessed as biochemical incomplete response

Abstract

Purpose: Although most patients with Papillary Thyroid Carcinoma (PTC) have a favorable prognosis, some continue to have elevated serum Thyroglobulin (Tg) levels after surgery and ¹³¹I treatment, classified as a Biochemical Incomplete Response (BIR). According to the 2015 American Thyroid Association (ATA) guidelines, 20% of these patients will develop a Structural Incomplete Response (SIR). Doctors often recommend ¹³¹I retreatment without imaging evidence, hoping for benefits. However, the effectiveness of this treatment strategy is still not supported by evidence. The aim of this study is to explore the potential benefits by comparing Tg levels and their changes in patients who were retreated with ¹³¹I and combining them with the results of follow-up assessment using Dynamic Risk Stratification (DRS).

Methods: This retrospective study collected clinical data from patients with PTC who underwent ¹³¹I therapy after surgery at two medical centers. All of them were evaluated for Biochemical Incomplete Response (BIR) after initial treatment. Based on whether they received ¹³¹I retreatment, they were divided into two groups: the retreatment group and the no-retreatment group. Our primary outcome measures included basic clinical data, suppressed Tg, change in Tg levels, and DRS stratification at the last follow-up. The potential benefits of ¹³¹I retreatment were analyzed by comparing the changes in Tg levels and the DRS distribution at the end of follow-up between the two groups.

Results: Patients with PTC assessed for BIR after initial therapy showed a greater reduction in Tg at short-term follow-up after ¹³¹I retreatment compared to the no retreatment group (0.84 ng/mL vs. 0.72 ng/mL). The one-sample Wilcoxon signed-rank test results were T = 6.001, p < 0.001 and T = 1.378, p = 0.168, respectively. At the end of follow-up, this treatment did not alter the DRS strata composition ratio (R = 3.06, p = 0.39). The area under the receiver operating characteristic curve for pre-ablation stimulated thyroglobulin (Ps-Tg) predicting non-remission status was 0.64 (95% CI: 0.55-0.73), with the highest Youden index (0.26) at a Ps-Tg cutoff of 20.5 ng/mL, corresponding to a sensitivity of 62.6% and a specificity of 63.3%. For the first suppressed Tg (sup-Tg) predicting non-remission, the AUC was 0.75 (95% CI: 0.67-0.84). The highest Youden index (0.40) was achieved at a sup-Tg cutoff of 1.85 ng/mL, corresponding to a sensitivity of 68.9% and a specificity of 70.8%.

Conclusion: For patients with PTC who received ¹³¹I therapy after surgery and are classified as BIR, ¹³¹I retreatment may accelerate the decline in Tg levels over time but does not change the DRS distribution. Only few of patients have structural and/or functional lesions detected on the whole-body scan after ¹³¹I retreatment. The Ps-Tg and first sup-Tg values after the first ¹³¹I treatment can help predict a non-remission state, with cutoff values of 20.5 ng/mL and 1.85 ng/mL, respectively.

Keywords: 131I treatment; Dynamic risk stratification; Papillary thyroid carcinoma; biochemical incomplete response.