Objective To explore the factors affecting the metabolic rate of ¹³¹I radiation dose in vivo in patients with differentiated thyroid cancer (DTC) after surgery and ¹³¹I treatment to provide guidance for radiation protection.

Methods A retrospective analysis was conducted on 72 postoperative patients with DTC (27 males and 45 females, aged 15–75 (42.79±14.23) years old) who were hospitalized in the department of nuclear medicine of three grade 3A hospitals from April to September in 2018. Among the above mentioned patients, 23 were from the Tenth People's Hospital Affiliated to Tongji University, 24 were from Renji Hospital Affiliated to Shanghai Jiaotong University School of Medicine, and 25 were from Union Hospital Affiliated to Tongji Medical College of Huazhong University of Science and Technology. The patients were divided into safety groups (48 h whole body radiation dose≤23.30 μSv/h) and risk groups (48 h whole body radiation dose>23.30 μSv/h) depending on whether the whole body radiation dose reached the safety standard 48 h after taking ¹³¹I, and the effects of various factors on the whole body radiation dose metabolism between the two groups were compared. The measurement data were compared by group t test or Wilcoxon rank sum test; the intergroup comparison of enumeration data was examined by chi-square test or Fisher's exact test. Univariate analysis was performed on each variable, and multivariate Logistic regression analysis was conducted for variables that were statistically significant in univariate analysis. The receiver operating characteristic (ROC) curve was drawn using each univariate indicator and combined indicators to determine the best critical value and diagnostic efficacy for predicting the efficacy of ¹³¹I.

Results The univariate analysis results of the comparison between the risk group and the safety group showed that the levels of thyroid iodine uptake rate at 2 h (t=−2.56, P=0.01), iodine uptake rate at 24 h (Z=−2.07, P=0.04), free triiodothyronine (Z=−2.83, P=0.01), free thyroxine (Z=−2.70, P=0.01), thyroglobulin (Tg) (χ²=6.80, P=0.01), and the presence of residual thyroid tissue in ultrasound examination (Fisher's exact test, P=0.03) significantly affected the metabolism of whole body radiation dose in patients with DTC after ¹³¹I treatment. The results of multivariate Logistic regression analysis showed that the 24 h iodine uptake rate (OR=1.27(95%CI: 1.03–1.57)), Tg level (OR=2.51(95%CI: 1.21–5.20)) influenced whole body radiation dose metabolism (P=0.03, 0.01). The higher the 24 h iodine uptake rate and the higher the Tg level, the lower the probability that the 48 h whole body radiation dose reached a safe level. The area under the ROC curve of the combined diagnostic index of 24 h iodine uptake rate and Tg level was 0.76(95%CI: 0.65–0.87). The sensitivity was 94.87%, the specificity was 46.88%, and the best critical value was −0.71.

Conclusions The 24 h iodine uptake rate and Tg level were the influencing factors of radiation dose metabolism in patients with DTC after ¹³¹I treatment. Therefore, these two factors can be used to establish a combined index to evaluate radiation dose, providing a basis for adjusting the length of hospitalization of patients.