The management of differentiated thyroid cancer using 123I for imaging to assess the need for 131I therapy

Background Follow-up of I-131 whole-body scanning after I-131 ablation is associated with potential stunning. Previous studies have suggested that, for scanning, I-123 is more sensitive than I-131 in identifying thyroid tissue, but its specificity when positive is less certain. Aim The use of I-123 as an imaging agent in place of serial I-131 imaging has been evaluated in the surveillance and treatment of differentiated thyroid carcinoma. Results A total of 186 studies in 136 patients with differentiated thyroid carcinoma were evaluated after total or near total thyroidectomy followed by I-131 ablation. In 125 studies I-123 scanning was negative and no I-131 therapy was given; four patients were positive on I-123 scanning but for other reasons no I-131 therapy was given. In 48/49 patients a positive I-123 scan was followed by positive I-131 therapeutic uptake. Only one patient failed to show positive uptake of I-131 when first treated and she subsequently demonstrated uptake on a second therapy. Conclusion High-dose I-123 imaging is the correct predictor of the I-131 post-therapy scan findings in most cases, at an administered activity that avoids stunning. As a diagnostic agent it is preferable to I-131 in differentiated thyroid carcinoma.