Abstract: Background and purpose: The concept of response-to-therapy assessment system (RTAS) was first proposed in the 2015 American Thyroid Association (ATA) guidelines. This study aimed to investigate the different short-term outcomes of thyroid ablation with low-dose (1 110 MBq) ¹³¹I for low-to-intermediate-risk thyroid cancer based on this system, and its probable influence factors. Methods: A total of 166 patients with differentiated thyroid carcinoma (DTC) ［50 males, 116 females; average age: (39.61±10.23) years］ undergoing first thyroid ablation with 1 100 MBq ¹³¹I from January 2015 to January 2017 were retrospectively observed. All patients underwent diagnostic whole body scan (Dx-WBS), and the stimulated thyroglobulin (sTg) was measured before ablation and 6-12 months after ablation respectively. According to above results, patients were divided into excellent response (ER) group and non-excellent response (NER) group. Clinical and pathological characteristics of 2 groups were compared using two-sample t test, Mann-Whitney U test and χ² test. Logistic regression was used to identify the influence factors for efficacy of ¹³¹I ablation, and the cut-off value was determined by receiver operating characteristic (ROC) curve analysis. Results: There were 127 patients in ER group and 39 patients in NER group. The successful ablation rate was 76.5% (127/166). Patients in ER group showed lower preablative sTg (ps-Tg) compared with patients in NER group. The ps-Tg levels were 1.5 (0.04-30.57) ng/mL and 17.6 (0.04- 21.52) ng/mL respectively (U=2 479, P<0.05). Male and patient with younger age were more prevalent in NER group (P<0.05). Logistic regression showed that ps-Tg, tumor size, the number of metastatic lymph nodes, gender, age and multifocality were independent predictors for efficacy of ¹³¹I ablation (OR: 0.361-2.875). The optimal cut-off value of ps-Tg identified by ROC curve analysis was 2.0 ng/mL. Conclusion: The patients with lower ps-Tg, smaller diameter of tumor, fewer lymph node metastases, male, younger age and single lesion are more likely to have ER within a short period following thyroid ablation. Ps-Tg of 2.0 ng/mL is identified as cut-off value for predicting the short-term therapeutic response, which has preferable sensitivity and specificity.

Key words: Differentiated thyroid cancer, ¹³¹I therapy, Response-to-therapy, Thyroglobulin