Patient selection and follow-up
The institutional review board of our university approved this retrospective study, and the requirement to obtain informed consent was waived (IRB approved no. 4-2021-0257). From September 2009 to June 2012, 245 patients were admitted to Severance Hospital for high-dose (≥150 mCi) I-131 ablation or treatment. Of those, 27 patients were excluded from this study as these patients received prior I-131 therapy, 46 patients were excluded for 200 mCi I-131 treatment for known metastasis, 29 patients were excluded due to metastatic sites found in post-RAI (radioactive iodine) WBS, and 45 patients were excluded due to loss of follow-up. Finally, a total of 98 patients were included who had no known metastasis, received initial I-131 therapy, and in whom follow-up TSH, Tg, and TgAb levels were detected for at least 2 years at approximately 6-month intervals as a routine clinical follow-up protocol. The TNM stage and serum TSH, Tg, and TgAb levels were all acquired on the ablation day, and lymphocytic thyroiditis statuses as determined by pathologic slides were recorded. Successful thyroid remnant ablation (gold standard) was defined as the absence of visualization of any structural lesions on neck ultrasound, undetectable serum Tg levels during levothyroxine administration (on-Tg) (<0.1 ng/mL), and TgAb level recordings <10 IU/mL between 6 months and 2 years after treatment.
Radioactive-iodine remnant ablation protocol
Before RAI remnant ablation therapy, all patients were instructed to discontinue levothyroxine for 5 weeks with triiodothyronine replacement for the initial 2 weeks. Patients underwent strict iodine restriction diets for two weeks before I-131 administration, which ended 3 days after RAI administration. TSH levels were confirmed to be at least 30 mU/mL before admission. All patients underwent two post-RAI WBS after hospital discharge. The first WBS was performed on the day of discharge (the third day after I-131 administration; Early scan) and on the seventh day after I-131 administration (Delayed scan).
Post-RAI whole body scan analysis
Early and Delayed post-RAI WBSs were acquired using a gamma camera equipped with a high-energy parallel hole collimator (Infinia, GE Medical Systems, Milwaukee, WI, USA). Early and Delayed scans obtained from a total of 98 enrolled patients were analysed. The count of these two scans were measured using a Xeleris workstation (GE Medical Systems) using the following method: A circular region of interest (ROI) was drawn on the neck on the Early scan to encompass the remaining thyroid, while the same size ROI was copy and pasted onto the neck of the Delayed scan. The same sized ROI was used for all patients, and the total counts in each ROI was recorded. Moreover, because whole body counts at early scans were higher compared to Delayed scans, post-RAI Early WBS scans were acquired faster than Delayed scans. To compare thyroid uptake in the Early scan compared to the Delayed scan, both Early scan speed and Delayed scan speed were factored into the analysis. This was done by first recording the scan speed on the DICOM header (Tag (0008,0008), which records scan speed as mm/sec), and then dividing neck ROI counts by scan speed for both Early and Delayed scans. Finally, Early scan counts were decay corrected to Delayed scan counts. This was done with the following formula:
![](https://myfiles.space/user_files/83062_751fab6dfaef2446/83062_custom_files/img1629198276.png)
Finally, the reduction ratio was calculated with the following formula: (Delayed WBS neck count - Early WBS neck count)/Early WBS neck count * 100.
Statistical Analysis
Continuous data were compared using the Mann-Whitney U test. Comparisons of categorical data were performed with the chi-square test. Correlation between post-RAI WBS neck uptake and laboratory findings were evaluated using Spearman’s correlation coefficients. Univariable and multivariable logistic regression analyses were performed to evaluate the predicted ablation success. P values less than 0.05 on univariable analysis were included in the multivariable analysis. With respect to post-RAI WBS parameters, receiver-operating characteristic (ROC) curves using the Youden index were used to determine the cut-off values for predicting ablation success. All statistical analyses were conducted using SPSS version 25.0 (IBM Corp., Armonk, NY, USA) and R version 4.0.3 (http://www.R-project.org). P values less than 0.05 were considered statistically significant.