Comparisons of I-123 diagnostic and I-131 post-treatment scans for detecting residual thyroid tissue and metastases of differentiated thyroid cancer

Objective We assessed the performance of 37 MBq I-123 as a diagnostic imaging agent in patients with differentiated thyroid cancer using comparisons with their corresponding high-dose post-treatment I-131 scans. Methods We reviewed diagnostic I-123 whole-body scans and post-treatment I-131 scans of 69 patients who underwent I-131 therapy for differentiated thyroid carcinoma (47 papillary and 22 follicular). Diagnostic scans were performed 24 h following the oral administration of 37 MBq of I-123. I-131 doses were administered 3 days after the I-123 diagnostic scans using 2.22–7.4 GBq (median = 5.55 GBq). All images for diagnostic I-123 scans and the corresponding post-treatment I-131 scans were interpreted by consensus of at least 2 experienced radiologists. They evaluated the accumulations of radioiodine in the following 5 sites: thyroid bed, cervical and mediastinal lymph nodes, lung, bone and others. The concordance rates between I-123 scans and I-131 scans were calculated. Results A total of 108 sites were identified on the post-treatment I-131 scans. Seventy-seven sites (71%) were also identified on the I-123 diagnostic scans. The concordance rates between I-123 diagnostic scans and I-131 post-treatment scans were high for thyroid bed and bone metastases (89 and 86%, respectively), while they were low for lymph node and lung metastases on post-treatment scans (61 and 39%, respectively). Conclusions Diagnostic scanning with relatively low dose I-123 is not always predictive of subsequent therapeutic I-131 uptake, especially for lymph node and lung metastases of differentiated thyroid cancer.