I-123 and I-131 scintigraphy discordance in pediatric thyroid cancer; effect of previous I-131 therapy.

Objectives To assess the discordance rate between pretreatment I-123 and post-therapy I-131 whole body scans in pediatric thyroid cancer patients.

Methods Medical record review identified 124 patients treated with radioiodine ( I-131) for thyroid cancer between 2004 and 2014 at a large pediatric hospital. 12 cases were excluded for incomplete data. 112 cases (87 initial, 25 retreatment) were analyzed for concordance of findings on the pretreatment andpost-therapy scans. Whole body scans were evaluated for abnormal uptake in three regions: central neck, lateral neck, and distant metastases. Discordance between scans was categorized as major if thepost-therapy scan showed abnormal uptake in a region not identified by the I-123 scan, minor if the I-131 scan had more extensive uptake in a region identified on the I-123 scan, and reverse if abnormal uptake on the I-123 scan was not confirmed on the I-131 scan.

Results There were 23 major (21%), 9 minor (8.0%), and 5 reverse (5%) discordances between the I-123 and I-131 scans. Major discordance was more frequent in patients being retreated 14/25 (56%) than initial therapy 9/87 (10%). New abnormal lung uptake was identified by I-131 scans in 8/25 (32%) of retreatment cases compared to 3/87 (3%) initial therapy cases. All 5 reverse discordance cases occurred with initial therapy.

Conclusions In 112 pediatric thyroid cancer patients, discordance between pre-therapy I-123 and post-therapy I-131 whole body scans occurred in approximately one-third of cases. Major discordances, half of which indicated new lung metastases, were more frequent in patients undergoing retreatment with I-131. Therefore, in pediatric patients with thyroid cancer undergoing retreatment with I-131, the post-therapy whole body scan may provide more information than a pre-treatment I-123 scan about the extent of iodine-avid disease.