MON-LB83 The Initial Dose of 131I as a Potential Independent Predictor for Residual/Relapsed Disease in Pediatric Differentiated Thyroid Cancer

In the guidelines for management of pediatric Differentiated Thyroid Cancer (DTC) 131I therapy is recommended for treatment of iodine-avid persistent locoregional disease that cannot be resected as well as iodine-avid distant metastases. To date, no consensus has been reached regarding the 131I dose for treatment of DTC in children. We report our institutional experience and highlight the initial dose of 131I as a potential independent predictor of residual/relapsed disease. Methods: We performed a retrospective analysis of all pediatric patients diagnosed with DTC between 2010 and 2018. The cohort included all patients up to 21 years of age, with minimal length of follow-up of 24 months. The risk stratication was done following the American Thyroid Association guidelines for pediatric DTC. We defined residual/relapsed disease as detectable thyroglobulin and positive anatomical lesions in imaging studies during the follow-up period. The log-rank test was used to evaluate disease-free survival. The P value was set at < 0.05. Results: Among 59 eligible patients, females were 69.5% (n=41) and males were 30.5% (n=18). The mean age at diagnosis was 16 years (9-21 years). All patients were alive at follow-up (median, 42 months; range 24 to 144 months). Fifty-eight patients had classic papillary thyroid cancer (PTC) and only 1 patient had follicular thyroid cancer. Among the patient with PTC, 39.6% (23/58) had follicular-variant PTC, 8.6% (5/52) had diffuse-sclerosing PTC and 17.2% (10/58) had other variants. Nineteen (32%), 30 (51%), and 10 (17%) had low-risk, intermediate-risk, and high-risk disease, respectively. Within the Low-risk group, 68% (13/19) received 131I. The mean initial dose was 60.9 mCi [26-150 mCi]. Eighty four percent (11/13) received ≤100 mCi and 27% (3/11) had residual/relapsed disease. Fifteen percent (2/13) received >100 mCi and none had residual/relapsed disease. Sixteen percent (1/6) of patients without 131I therapy had residual/relapsed disease. (P=0.48) Within the Intermediate-risk group, all 30 patients received 131I. The mean initial dose was 97.5 mCi [27.3-215 mCi]. Sixty percent (18/30) received ≤100 mCi and 38.8% (7/18) had residual/relapsed disease. Forty percent (12/30) received >100 mCi and 16.6 % (2/12) had residual/relapsed disease. (P=0.15) Within the High-risk group all 10 patients received 131I. The mean initial dose was 159.9 mCi [129.3-384 mCi]. Fifty percent (5/10) received ≤150 mCi and 60% (3/5) had residual/relapsed disease. Fifty percent (5/5) received >150 mCi and 20% (1/5) had residual/relapsed disease. (P=0.2) Conclusion: There are no statistical differences of disease-free rate between the initial dose of 131I among all risk categories. However, the use of more than 100 mCi in the intermediate-risk category and more than 150 mCi in the high-risk category may be recommended.