Early prognostic factors at the time of diagnosis of bone metastasis in patients with bone metastases of differentiated thyroid carcinoma

Objective: Bone is the second most common site of distant metastases from differentiated thyroid cancer (DTC). Patients with bone metastases were associated with poor clinical outcomes; however, their clinical courses are heterogeneous. The aim of this study is to evaluate early prognostic factors of patients with bone metastases from DTC at the time of diagnosis of bone metastasis. Methods: This retrospective study included 93 patients with bone metastases from DTC. We defined 'Pre-RAIT group' as patients whose bone metastases were detected before initial RAIT. The 'post-RAIT group' was defined as patients whose bone metastases were detected after initial RAIT or during the follow-up period. Results: Median age was 55.4 years, and 55 patients (59%) had papillary thyroid cancer. Patients in the pre-RAIT group (n = 32) demonstrated significantly poorer overall survival (OS) (HR = 1.86, P = 0.04) than those in the post-RAIT group. There was no significant difference in the OS according to the initial RAI avidity among all patients (P = 0.18). RAI-avid bone metastases had better OS only in the pre-RAIT group (HR = 0.23, P = 0.01) but not in the post-RAIT group. In the post-RAIT group, older age (>45 years), elevated serum thyroglobulin (Tg) level (>250 ng/mL), and the presence of skeletal-related events (SREs) were significantly associated with poor OS. RAI avidity was not a significant prognostic factor in the post-RAIT group (P = 0.33). Conclusions: Patients whose bone metastases were diagnosed before initial RAIT demonstrate a poorer prognosis. RAI avidity is an early prognostic indicator in the pre-RAIT group. Old age, higher serum Tg levels, and SRE are associated with poor survival outcomes in the post-RAIT group.