The Use of 123I in Diagnostic Radioactive Iodine Scans in Children with Differentiated Thyroid Carcinoma

Background: Adult studies have shown that iodine-123 (I-123) is as effective as I-131 in detecting metastatic disease in patients with differentiated thyroid carcinoma. However, the type and administered activity of radioiodine used for diagnostic imaging of metastatic thyroid cancer has not been well studied in children. Here we describe our institution's experience with using I-123 in diagnostic radioiodine scans in children with differentiated thyroid carcinoma. Methods: Every patient with differentiated thyroid carcinoma who completed diagnostic scanning followed by radioiodine therapy at our institution over the past 8 years was included in this retrospective chart review. Patient age, sex, presentation of thyroid disease, past medical history, thyrotropin, thyroglobulin, and antithyroglobulin antibodies were recorded. A single nuclear medicine radiologist evaluated all scans. Results: Thirty-three subjects completed 37 pairs of scans at a mean age of 13.4 years (range 6-17 years). The majority of subjects were female (81%) and had papillary thyroid cancer (91%). For diagnostic scanning, 5 received 2 mCi of I-131, 21 received 2 mCi of I-123, and 11 received 3 mCi of I-123. There was no statistically significant difference in rate of discordant scan pairs when comparing I-131 and I-123 (20% and 23% respectively, p=0.9). The detection of metastatic pulmonary disease on diagnostic scanning was not improved by increasing the dose of I-123 from 2 mCi to 3 mCi (10% rate of missed lung detection with 2 mCi I-123 vs. 20% with 3 mCi I-123). Conclusions: I-123 is effective for use in diagnostic radioactive iodine scans in children with differentiated thyroid cancer. The primary advantages of using I-123 include decreased radiation exposure and avoidance of stunning. However, in children there is a possibility of missed detection of metastatic pulmonary disease.