Treatment intensity and control rates in combining external-beam radiotherapy and radioactive iodine therapy for metastatic or recurrent differentiated thyroid cancer.

Background: To evaluate the treatment outcomes of external-beam radiotherapy (EBRT) with or without radioactive iodine therapy (RAIT) for metastatic or recurrent lesions of differentiated thyroid cancer (DTC).

Methods: Between August 1997 and March 2018, 73 lesions (distant metastases, 50; regional lymph-node metastases, 17; postoperative tumor-bed recurrences, 6) in 36 patients that had received EBRT with or without RAIT were reviewed. Doses of EBRT were 8-70 Gy (median 40 Gy). Seventeen patients received RAIT after EBRT.

Results: Median follow-up time of imaging studies was 14 months (range 1-110 months). Two-year overall survival rates and control rates of EBRT sites were 71% and 62%, respectively. Two-year control rates for EBRT of < 30 Gy (n = 7), 30 Gy (n = 13), 31-49 Gy (n = 25), 50 Gy (n = 20), and > 50 Gy (n = 8) were 0%, 56%, 53%, 79%, and 100%, respectively. There were statistically significant differences in control rates between < 30 Gy and 30 Gy (p = 0.003), and between 50 Gy and > 50 Gy (p = 0.037). Control rates of > 50 Gy were significantly better compared to ≤ 50 Gy (p = 0.021). Two-year control rates with (n = 28) and without (n = 45) post-EBRT RAIT were 89% and 45%, respectively (p = 0.009). In multivariate analysis, EBRT of > 50 Gy and post-EBRT RAIT were significant independent factors for favorable control of EBRT sites (hazard ratio [HR], 5.72; 95% confidence interval [CI], 1.21-27.1; p = 0.028 and HR, 2.98; 95% CI, 1.28-6.98; p = 0.012, respectively).

Conclusion: EBRT of > 50 Gy and post-EBRT RAIT appeared to be useful for long-term control of EBRT sites for metastatic or recurrent lesions of DTC.