Initial Evaluation of Radioiodine Therapy Using Imaging for Long-term Prognosis in Thyroid Cancer: A Retrospective Study.

[OBJECTIVE] Radioactive iodine (RAI) is an established therapy for thyroid cancer (TC). The aim of this study was to evaluate the prognosis and disease characteristics based on initial RAI therapy using image findings.

[METHODS] We reviewed the clinical records of 290 TC patients who received initial therapeutic RAI therapy following total thyroidectomy for ablation and treatment between 2010 and 2021. Patients were classified into 3 groups based on the 131I whole-body scintigraphy (WBS) combined with morphological imaging records: non-DM, RAI-non-avid DM, and RAI-avid DM. Progression-free survival (PFS) and overall survival were assessed in each group. Normalized indices were calculated automatically using dedicated software for each patient on WBS. 131I-avid lesions were defined as areas with uptake exceeding the mean + 2 SDs of the reference regions of interest placed at the head-cervical, thoracic, and abdominal-pelvis regions, Watabe proposed.

[RESULTS] The 5-year and 7-year survival rates for the non-DM, RAI-non-avid DM, and RAI-avid DM groups were 98% to 95%, 86% to 78%, and 82% to 82%, respectively. Average count of 131I-avid area was identified as a significant prognostic factor for PFS, being independent of age, distant metastatic pattern, histological subtype, and T classification. Retreatment was preferably performed within 2 years after the initial RAI therapy, as this period appears to be critical for efficacy. Additionally, the site of metastases was a strong prognostic determinant.

[CONCLUSION] Precise imaging evaluation of the initial RAI therapy may help predict the prognosis during the first 5 years, contributing to more effective comprehensive treatment approaches for long-term management.