Purpose: Using 123I for diagnostic purposes avoids the risk of stunning for subsequent radioiodine treatment and affords an excellent image quality. In this study we assessed the role of 123I in comparison with 131I post-treatment imaging in patients with thyroid cancer.
Methods: We compared a total of 292 123I scans with their corresponding post-treatment 131I images. Patients received a therapeutic dose of 131I following diagnostic scanning with 50-111 MBq of 123I. All patients were in a hypothyroid state (>30 microIU/l) before radioiodine administration for either diagnostic or therapeutic purposes.
Results: In 228 out of 263 patients with a positive diagnostic scan, 123I whole-body scan findings were concordant with those of corresponding post-treatment 131I images (concordance rate 87%). However, there were 44 additional foci of abnormal uptake on post-treatment 131I scans in 22 discordant cases with no impact on therapeutic management of the patients. In 13 patients, there was at least one new site on post-treatment images that had been missed on pretreatment 123I images. Twenty-nine patients with a negative diagnostic scan were treated with 131I owing to a high serum thyroglobulin level (range 11.3-480 ng/ml). Radioiodine uptake sites were seen in eight post-treatment scans. In 21 pairs of whole-body scans, both the pre- and the post-treatment scan were negative (concordance rate 72.4%).
Conclusion: 123I scanning is comparable to high-dose 131I post-treatment imaging in thyroid carcinoma patients, and 123I offers excellent image quality as a diagnostic agent. It avoids disadvantages such as stunning before treatment and delivery of a high radiation dose to patients.