Objective To investigate the efficacy response and influencing factors of higher dose ¹³¹I in patients with metastasis-free intermediate-to-high risk differentiated thyroid cancer (DTC).

Methods The clinical date of 378 patients with intermediate-to-high risk DTC who attended in the First Hospital of Shanxi Medical University from January 2018 to December 2020 were retrospectively analyzed. The patients included 103 male and 275 female with a median age of 45(13–85) years. All patients were given the first ¹³¹I remnant ablation and/or adjuvant therapy at a dose range of 3.70–5.55 GBq. Postoperative residual thyroid ⁹⁹Tc^mO₄⁻ image was performed before ¹³¹I treatment, and ¹³¹I post-treatment whole body scan (Rx-WBS) was performed 2–7 days after treatment. All patients were followed up at least 6 months with a median follow-up time of 16.3 months. The efficacy response evaluation system was evaluated according to the 2015 American Thyroid Association assessment system, and its influencing factors were analyzed. Spearman rank correlation analysis was used to evaluate the correlation between ⁹⁹Tc^mO₄⁻ image target/non-target (T/NT) ratio and Rx-WBS score.The Mann-Whitney U test, χ² test or Fisher's exact probability method was utilized to analyze whether the difference between the excellent response (ER) and non-excellent response (nER) groups was statistically significant for all observed indicators. Binary multivariate Logistic regression was used to analyze the independent influencing factors of prognostic ER, receiver operator characteristic (ROC) curves were utilized to obtain optimal diagnostic thresholds.

Results Stistically significant differences were observed in maximum tumor diameter, postoperative stimulated thyroglobulin (psTg) level, N stage, and interval time between the two groups of ER and nER (Z=−7.127, −2.702, Fisher's exact probability method, χ²=6.783; all P<0.05). The differences were statistically insignificant for age, sex, capsule involvement, tumor multifocality, T-staging, risk of recurrence stratification, first dose of ¹³¹I treatment, thyroid-stimulating hormone (TSH) level, urinary iodine level, ⁹⁹Tc^mO₄⁻ image T/NT values, and Rx-WBS score (Z=−1.505 to −0.664, χ²=0.064–5.501, Fisher's exact probability method; all P>0.05). Spearman rank correlation analysis showed a moderate correlation between ⁹⁹Tc^mO₄⁻ image T/NT values and Rx-WBS imaging scores (r=0.530, P<0.001). A multivariate Logistic regression analysis showed that psTg level was the independent risk factor of ER. The ROC curve analysis showed that the optimal cut-off value of ER was 5.90 ng/ml, the sensitivity was 83.76%, and the specificity was 59.81%.

Conclusions Patients with intermediate-to-high risk DTC treated with higher dose of ¹³¹I had the same good ER proportion regardless of the amount of residual thyroid indicated by thyroid ⁹⁹Tc^mO₄⁻ imaging. Patients receiving ¹³¹I treatment within 2 months after surgery may obtain better ER proportion. Meanwhile, psTg was an independent risk factor for predicting ER proportion in patients with metastasis-free intermediate-to-high risk DTC.