Effect of administrated activity, admission number and TSH level on radiation retention curve of patients taking iodine-131 therapy for differentiated thyroid carcinoma: Looking beyond established regulations

Introduction: Retention of I-131 in the body of patients with differentiated thyroid carcinoma (DTC) has a direct effect on therapeutic outcome of radioiodine therapy. There are several factors that may influence retention time of radioiodine in the body of these patients. In this study we are going to assess effects of administered radioiodine activity, serum thyroid stimulating hormone (TSH) level and also admission/administration number on the retention of I-131 in the body of DTC patients. Methods: Number of 92 DTC patients, with different TSH levels ranged from 32 mIU/ml to 250 mIU/ml, was treated with different I-131 activities ranged from 1850 MBq to 7400 MBq. They received one, two or three sessions of radioiodine therapy based on severity of cancer. Post-therapy dosimetry for all the patients was performed immediately, 6 hours, 12 hours, 24 hours and one week after the administration of  activity to calculate clearance rate of the I-131. Results:Basedon our observations,there wasa direct correlation between administered radioiodine activities to the patients and clearance rate of the activity from their bodies. Also, those patients who had been admitted for the third time to receive their third radioiodine administration had higher clearance rate as compared to those who had their first or second admissions. TSH level above 32 mIU/ml did not show any effect on clearance rate of I-131. Conclusion: Based on our findings, clearance rate of I-131 is affected by amount of administered radioiodine activities to the patients and number of radioiodine therapy sessions, but serum TSH level above 32 mIU/ml had no effect on this quantity.