Prognostic values of the cervical lesions absorbed dose in the radioactive iodine treatment of differentiated thyroid carcinoma

Objective To evaluate the dose-response relationship of thyroid remnants and differentiated thyroid carcinoma (DTC) cervical metastases in the radioiodine treatment of DTC. Methods Post-therapeutic iodine-131 whole-body scintigraphy and SPECT/CT imaging were performed in 22 patients with DTC, including 10 males and 12 females, of the Nuclear Department of Third Affiliated Hospital of Sun Yat-Sen University. The ages of the patients were between 21 and 59 years, with a median of 39.5 years. Images were acquired from iodine-131 whole-body scintigraphy and SPECT/CT at multiple time points after treatment. The absorbed doses (ADs) of thyroid remnants and DTC metastases and the mean individual ADs were calculated using these images. Lesion response was determined using the 2015 American Thyroid Association Management Guidelines for Adult Patients with Thyroid Nodules and Differentiated Thyroid Cancer. The differences in lesion AD of each observed group were analyzed using the Mann- Whitney U test. Moreover, the receiver operating characteristic (ROC) curves were used to test the performance of the estimated AD for prognostic assessment. Results All (28/28) thyroid remnants and 34.8% (8/23) of DTC metastases responded completely. The lesion ADs of the completely responded DTC metastases (M=79.3 Gy) were significantly higher than that of the incompletely responded lesions (M=29.8 Gy) (Z=-2.195, P=0.028). The ROC curve analysis indicated that the estimation of lesion AD, which had an area under the curve of 0.783 (Z=-2.195, P=0.028) for DTC metastases, may be a prognostic factor for the prediction of lesion-based iodine-131 therapy response. The corresponding lesion AD threshold value for correctly predicting the complete response of metastatic lesions was 70.6 Gy. The mean individual ADs of clinically relieved patients, which had an area under the curve of 0.823 (Z=-2.285, P=0.022), were also significantly higher than that of the clinically nonrelieved patients. Conclusion Completely responded metastases demonstrated higher AD than the incompletely responded ones. The AD of iodine uptake tissue and the average AD of patients are possibly valuable to predict the response to iodine therapy. Key words: Single photon emission computed tomography computed tomography; Differentiated thyroid carcinoma; Iodine radioisotopes; Dosimetry; Prognosis assessment